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Heart Theft: The Comprehensive Case Files By Kaeya Alberich

Summary:

After his return to Mondstadt four years following the death of his father, Diluc never quite seemed the same, as Kaeya soon came to realize. Though, this change was at least constant, and he grew to accept this distant personality.

That is, however, until Diluc suddenly reverted back a bit. His smiles showed more often, and his shoulders seemed a bit lighter. Kaeya could have almost grown to appreciate this change. That is, until he found out it had only been caused by a series of affectionate letters exchanged between the man and an anonymous lover. Now, Kaeya was only irritated. Irritated, and incredibly determined to find out who in Teyvat could have possibly stolen Diluc's coal heart.

Chapter 1: Initial Deliberation

Notes:

(See the end of the chapter for notes.)

Chapter Text

Diluc Ragnvindr was many things, Kaeya had observed. He was steadfast in his resolve, he was a wine tycoon who hated wine, he was a raging torrent to his enemies whilst a gentle wind to his allies.

And he was terrible at keeping secrets.

Which was why Kaeya had now ordered his sixth Death After Noon just to watch him work. There was something different about the man this evening—he spun his mixer one time too much, tied up his hair one inch too high, and rolled back his sleeves one fold too far. 

Or perhaps Kaeya was imagining it, he figured, sloshing around the liquid in his glass as his mind began to swirl as well.

Even so, he couldn’t simply let it go. Diluc was different. And not in the ‘gone-for-four-years-and-completely-changed’ way, either. He was quiet, amiable, and utterly unfazed by Kaeya’s teasing jabs.

Completely out of character.

“Say, Master Diluc, do you remember that time when we were children and you tried making a flower bouquet out of weeds for Father’s Day?”

“They were grapevines, not weeds. And you encouraged it as well—even caught crystalflies to fill the bouquet,” Diluc corrected. The thin line of his lips twisted in fond remembrance. “Why do you bring it up?”

He took a swig from his glass. “Just making sure you remembered…” And that you haven’t been replaced by a—much nicer—clone, he mused.

The redhead returned to his work without a word, running his damp towel through empty glasses until they sparkled like crystal chunks in the sun.

No remark, no scowl, no scorn.

There was something deeply, terribly wrong with Diluc Ragnvindr. 

 


 

Kaeya had made it a practice to keep a keen eye on Diluc’s schedule—not because he intended to stalk the man, but because he was determined to investigate his drastic change in character. He would never directly ask Diluc if anything was wrong, no, because the stubborn redhead would likely only deny it and shut out whatever momentary kindness he held in his heart for Kaeya.

So, the knight resorted to manipulating his own work schedule in order to observe his brother as much as possible: patrol routes through the Whispering Woods on Sunday afternoons when he’s on his weekly stroll, watch guard in Springvale on Wednesday mornings when he drops off fresh grape juice and bread for the kids, assignments nearby Dawn Winery to catch him entering and leaving.

But not long later, his streak had run out. The main city patrol unit had dwindled as several knights were dispatched on an expedition, and Jean insisted Kaeya take over for the day. In order to not arouse suspicion, he could do nothing but simply comply without question. If the grandmaster ever found out he was only working so diligently to get closer to Diluc, he would never hear the end of it from her.

So, he now leaned against the wall Good Hunter with a half-eaten mushroom skewer in his mouth, his eyes casually glancing over the area. Children played around the fountain, bards strolled around with their lyres, and families sat down to eat at tables.

Which was the majority of what happened within the walls at any given time. Kaeya sighed, picking at his teeth with the skewer to pass the time. He would have swung by the Angel’s Share at this point, but Diluc wasn’t on shift today, so there was no point in even going. Instead, he dawdled around aimlessly, cursing Jean for leaving a distinguished captain with such a mundane job.

Then he saw red.

Or rather, a slight blur of scarlet poking through the city gates. It was impossible not to spot, even when swallowed up in black clothes. 

Kaeya perked up, straightening up from his spot and drawing nearer to the entrance. He spotted the redhead speaking to Katheryne at the Adventurer’s Guild, so he stopped dead in his tracks and hid in an alley. 

He peeked over the corner to where Diluc stood, seeming very animated as he spoke. Katheryne smiled and nodded in response before reaching from behind her to pull out an envelope. She chuckled a bit as she handed it to him, who tore it open on the spot and began reading the paper inside.

And Kaeya swore there was something in the atmosphere distorting his vision.

Because Natlan would sooner freeze over before the captain would witness Diluc Ragnvindr smiling. 

It was rare even to see him with a close-lipped smirk, let alone a large, toothy grin. He even laughed softly as he read the paper, his eyes raking over it with fondness.

Kaeya gawked at the sight in bewilderment for a few more moments before deciding that he was not, in fact, hallucinating. And so, he slinked away from the shadows and approached the man, feigning casual notice.

“Oh?” he started, bounding up to the redhead and slinging an arm around him. “Is that Master Diluc cracking a grin? Whatever is the miraculous occasion for such a rare sight?”

Diluc’s sweet smile melted into a scowl at the contact, and he rolled his eyes. “Mind your own business, Sir Kaeya. Aren’t you supposed to be on patrol right now?”

“My, don’t tell me you stalk my schedule now?” Kaeya quipped, then bit his tongue at the irony of what he’d just said.

He drew out a long sigh and massaged his temples. “No, I doubt you would remember, but you were rambling on about how ‘unfair’ it was when you were drunk off your ass last night. How is that hangover treating you, by the way?”

The captain scoffed, turning up his chin in faux offense. “Perfectly fine, mind you.” It was a bold-faced lie, as he was still nursing a piercing migraine, but he shoved it aside regardless. “Now, back to the pressing matter at hand. What is this?” he gestured to the letter— “A big fat check or something?”

Before Diluc could react, Kaeya snatched the paper from his hands and maneuvered around whenever he reached for it.

‘March 11th—’ ” he began reading aloud, ducking as the redhead made another grab for it— “ ‘My Dear Phoenix, I adored the gifts you sent over last month since I really didn’t expect them. I decided to return the favor, and if I got the timing right, your gifts should arrive just in time for White Da—’ Wait—” he stopped dead in his tracks, racking over the words on the page. “Barbatos above, is this a fucking love letter?”

Diluc finally managed to yank back the letter, and he stuffed it into his coat pocket, his face beet red. “I told you to mind your own business,” he grumbled as he hid behind his sleeve.

The captain nearly choked. “You’re not denying it, though? So it is a love letter!” His mind raced, mainly in utter shock at the fact that Diluc was in a relationship. “When the hell did this happen? Do I know this person? How long has this been going on? Is it serious? Have you guys…” He made lewd finger gestures, to which Diluc nearly gagged.

“Enough with the incessant yammering! This is why I didn’t tell you in the first place. I merely wanted to keep this to myself without being bombarded with questions. I’d rather not have the entire city know about my sex life.”

Kaeya huffed in dismay. “Oh, come on! I deserve to know about these things. You’d think, as your brother, I’d be the first to—”

“We're not —” Venom coated his voice in stinging acid, but he bit back the retort, gnashing his teeth together and taking a deep breath as his nostrils flared—“Look, just don’t worry about it, alright? I’m not ready to divulge such details quite yet, since the situation is a bit… precarious. How about I don’t pry into your personal life, and you don’t pry into mine. Fair?”

His heartbeat caught in his throat, aching and swollen. There was once a time when they were each other’s personal lives. But time eroded their bond, and any fond memories soured with the sound of fire clashing against ice one storming night long ago. The thought put a bitter taste in the knight’s mouth, but he wiped away any disdain etched on his face with a flashy smile. “Right. I suppose that’s as fair as you can get,” he reasoned lightheartedly with a slight chuckle. 

Diluc sighed and nodded, bidding him farewell and turning back towards the front gate without another word. Kaeya watched as he slowly became a red haze in the distance, then disappeared altogether.

And despite the heat in Diluc’s voice just moments ago, the knight couldn’t help but notice a slight vigor in his steps as he reread that letter on his way out.

 


 

Kaeya Alberich made a solemn promise to himself not to pay any mind to the sudden developments in his brother’s love life. They each made a silent decision not to be in each other’s lives years ago. So, if no one else knew, he shouldn’t either. He held no special place in the redhead’s memories.

So, he strolled into Angel’s Share with not a hint of ill intent, only looking to wind down after another monotonous patrol shift. 

“Your Death After Noon, Sir Kaeya.” Diluc set down the drink as soon as the captain took a seat.

“Oh my, you know me so well. I feel like a true regular now,” Kaeya mused, circling the rim of the glass with his finger as he flashed a coy smirk.

“It’s not like you have any variety in your drink choices,” the redhead retorted flippantly.

“Well, actually , I was planning on trying out that new special today instead.” He pointed to the wooden sign beside the menu and slid his drink back over. “So I won’t be needing this.”

“Are you serious…” He took back the glass with a scowl of thinning patience stretched over his lips. “I have other customers, you know. Do you just want to give me more work?”

“Who, me? Inconvenience you in any way? I would never. I just wanted to—how did you say it? ‘Have variety in my drink choices’?”

Diluc merely gritted his teeth as he dumped the drink and got started on a new one. In one fluid motion, he poured several different syrups and juices into his shaker, adding liquor and ice before closing it up. He always moved with practice ease as he shook the metal mixer, like he could do this backward in his sleep. And despite the clear annoyance in his tone, he took on the drink with grace and swiftness. Kaeya was tempted to order five more just to see if he would tire.

Diluc then poured the magenta liquid into a cocktail glass rimmed with sugar and a sliced strawberry. He garnished it with an orange twist before finally sliding it across the counter. “Your Bleeding Heart Daiquiri, Sir Kaeya,” he sighed, a frown etching lines on his face.

“Why thank you, Master Diluc.” As he sipped on the concoction, a sweet blend of strawberries and wolfhooks left a satisfying tart aftertaste on his tongue. He usually never went outside his comfort zone when it came to drinks—this time only doing so to prove a point—but this was a pleasant surprise.

“Huh… It’s actually good,” Kaeya noted, taking another sip.

“Why are you acting surprised? If it wasn't good, it wouldn’t be on the menu.”

“Oh, I don’t doubt your skills, Diluc. I only figured you would slip me poison if I annoyed you again.”

“Don’t be ridiculous.” The redhead scoffed and blew his bangs out of his eyes a few times before groaning in irritation and undoing his hair ribbon altogether. He gathered his loose locks into a higher ponytail before tying back everything in his face.

And, of course, Kaeya was quick with a sly remark. “What a fresh face! I don’t think I’ve even seen your eyes in…” his quip trailed off as his gaze wandered to Diluc’s ear, where one shining earring dangled downwards. “What the hell…” He cleared his throat and spoke up. “I didn’t know you had your ears pierced, Master Diluc.”

He furrowed his brow for a moment before touching his earring in realization, and Kaeya could have sworn a faint flush was blooming across his face. “Oh, um, no, I didn’t. I poked my ear with a needle to wear this. Does it look bad?”

He could almost laugh. “You did it yourself for this? It looks fine, but… what possibly could have prompted you to…” The captain grew lost in thought. Diluc wasn’t the type of person to make spontaneous style changes. It was a rare occasion for him to even wear a lighter color of attire. But this? For him to commit to a piercing when he could have just as easily purchased clip-ons if he wanted to? He couldn’t have possibly made such a decision on his own. It couldn’t have been any more out of character.  

Kaeya’s eyes widened the longer he stared at the jewelry, his mind flooding with overwhelming realization. It couldn’t have been his own decision. “Don’t tell me… Is this the infamous aforementioned White Day present?” His lips quirked upwards into a smirk as Diluc sputtered and hid his ears behind his bangs.

“N—no…” the redhead tried to defend.

He was not buying it. “Oh really? So you’re telling me that you decided to get a piercing out of the blue, imported this thing from gods know where—because that craftsmanship is definitely not from Mondstadt—and haphazardly poked your ear with a sewing needle just to wear it?”

“Yes.”

“Bullshit.”

“What?”

“Come on, Diluc. You’re a terrible liar. Just admit that it was a present. It’s nothing to be ashamed of.”

Diluc averted his gaze and fiddled with his cleaning rag before clearing his throat. “Fine, yes, it was a gift. Now, could you please drop the subject?”

Oh, but Kaeya was only getting started. His apprehension melted away as intrigue took hold of him like a firm hand guiding his heart. “No, no, my interest has piqued now. It’s quite adorable, actually. I’m just curious… Who, pray tell, could have possibly captured the heart of Mondstadt’s uncrowned king?”

He grumbled and turned his head further away from the knight, unintentionally showing off his red-tipped ears and that bright ruby hanging down one of them. “What happened to us minding our own business?”

“Please, that was before your mysterious lover brought you such a flashy present. Marking their property like that? You may as well have tattooed their name across your forehead. It’s right in my face, too, so I believe it does count as my business.”

“Kaeya, let it go,” Diluc insisted as his face grew to an even deeper shade of red.

“Won’t you tell me who your lover is?”

“No.”

“Oh, come on, it isn’t a big deal.”

“No.”

“Please?”

“I said no.”

“Pretty please?”

Diluc rolled his eyes. “How old are you? You haven’t pleaded like this since we were children fighting over leftovers.”

“Exactly! I’m really wounding my ego here, so can’t you just give me some grace and tell me who you’re with?”

“Why do you care so much?”

“Because—” we’re family. Kaeya had wanted to say that. But they weren’t. Not anymore. Not after he lost the right to call him that. He could see it in the redhead’s eyes; he saw him as nothing more than a stranger. The knight bit back the bitter sting in his throat with a dry laugh as he took another sip of his drink. “I’m just naturally curious. You don’t get to be a captain without having a drive for problem-solving,” he bounced back, flashing a quick smirk for good measure.

“I think you ought to learn the phrase ‘curiosity kills the cat,’” Diluc deadpanned, unamused.

“I think you ought to learn to be more sure of yourself. How would your partner feel if they knew you were trying to keep them a secret? Honestly, if I was you, I’d be singing from the rooftops about how deeply I'm in love.” He accentuated the last word, almost humming it. This brought a pointed huff from the other man, to which Kaeya chuckled.

“My partner is well aware that I’m a reserved person. They don’t mind.”

“Are you reserved or just embarrassed? Have you entangled yourself in a forbidden romance? Or, better yet, has Donna finally had her way with you?”

Diluc nearly choked on his own breath and grimaced. “No. I can assure you, if anything, I am not dating Donna.”

Kaeya broke into a full laugh. “Oh my, now that she’s off the table, I’m sure she would be heartbroken at the fact that you’re no longer an eligible bachelor.”

“Right. And I’d like for that information to remain in the dark, Sir Kaeya. Now, if you wouldn’t mind, you’re holding me up from my job.”

“Wait, this conversation is far from—” his voice died in his throat as the redhead turned away to serve other customers, not even giving him a second thought. Dejected, he absentmindedly swirled his straw in his glass and watched the small glimmer of Diluc’s earring through his hair. Its square-cut crimson gem hung from a chain and rested on his shoulder, swaying every time he took a step. From the size of the stone alone, Kaeya could tell it cost a fortune—which made the situation all the more compelling.

“Oho, don’t tell me you’re drinking alone, captain?” a jubilant voice chirped from beside him.

Kaeya hardly got the chance to respond before a green bard dropped into the barstool next to him. “Good evening, Venti,” he mumbled, eyes still wandering.

“Please, is that any way to greet your favorite drinking buddy?” he jested, playfully nudging the captain.

“‘Favorite’? I don’t recall you suddenly dying your hair purple and changing your name to Rosaria.”

Venti gasped in mock offense and put both hands over his heart. “Oh, you wound me so, Sir Kaeya. I very well could dress up like a vampire and go to church, but could Rosaria play a tune like I can?”

He let out a dry chuckle as he swirled the drink in his hand. “Mm, I suppose not.”

The bard’s smile dissolved, and he narrowed his gaze toward the knight. “What ails you, Kaeya?” he asked, his voice softening around the edges.

“Hm? Nothing, I assure you,” he muttered over the rim of his glass.

“Really? You haven’t looked at me once since I got here.”

Kaeya perked up and turned towards the bard. “What? Perhaps I’m just enjoying this new drink way too much. How about I order one for you, too?”

Venti’s eyes immediately lit up at the mention of alcohol. “Wait, are you serious?”

“Of course.” He motioned over for Diluc. “Oh, barkeep! He’ll have what I’m having—on me.”

Diluc nodded in acknowledgment and returned to his work, earring still jostling around with every movement.

“Wait a minute.” Venti blinked and cleared his throat. “You didn’t just do that to change the subject, did you?”

“I haven’t a clue what you’re talking about.” Kaeya could swear that those rubies came from Liyue. Or perhaps Inazuma? Or, maybe even the mines in Snezhnaya, or the underwater caves in Fontaine, or—

A hand waving in front of his face tore him from his train of thought, and he side-eyed Venti with distaste. “Do you want attention this badly?” he teased.

“No, I want you to tell me where you’re staring off into.” The bard wore a soft pout as he crossed his arms.

The captain could hardly resist the look. He sighed, “Well, our dear Diluc over here has had recent… developments in his life… and I’m merely trying to figure out what’s going on.”

His pout curled upwards into a smirk, and he could hardly hold back his laughter. “Oh, Kaeya, are you trying to figure out who he’s dating?”

“What?” He did a double take, the question replaying in his head like a scratched vinyl. “How did you know?”

“I mean, even without context, your implications said enough. Do you really not know?”

“Do you know?” Kaeya asked incredulously. Was this… common knowledge? Doubts began to creep up his spine and burrow into his nerves. There was no way Venti knew before him. Right? 

“Why, of course. The wind knows all.” Venti flashed a snide smirk and put a finger to his lips as he winked.

The knight simply rolled his eyes. “Alright, so are you gonna fill me in or not?”

He giggled and drummed his fingers on the tabletop. “A bard who spins tales of secrets not his own, gets his tongue turned to rubber and his hands reduced to bone.”

“So… what you’re saying here is you’re going to sit there and do nothing to help.”

“Yep!”

“Fine. You pay for your drink, then.”

Venti’s sly expression instantly melted into a look of pure terror. “No, wait! I’m sorry! Please, I’ll help!”

Now was Kaeya’s turn to be smug. “Oh? Is it that simple to win you over?”

He pouted again and turned up his chin. “No… I’m not just going to hand you the answer. Where’s the fun in that? Besides, I don’t want to give you the wrong idea. I found out everything myself, Diluc never told me. So, you it's only fair for you to do the same.”

A sharp knot in the knight’s chest he refused to acknowledge had unraveled, loosening his nerves. Pinching thoughts had incessantly bit at the back of his mind, convincing him that his brother would rather open up to a drunk bard than him. Yet now, like a breath of relief, Venti soothed his doubts without even realizing it.  

“How did you figure it out so quickly?”

“I mean, really, it’s quite obvious. I’m surprised you haven’t realized yet.”

Kaeya ground his teeth and clicked his tongue, his patience running thin. “Well, perhaps it would be easier if you would just give me a straight hint, then?”

“I mean… I guess I could—”

The sound of a glass slamming on the counter jolted Venti out of his thoughts, and he turned to find Diluc sliding a tall drink to his side.

“Oh, thank you, Master Diluc—”

“Ah—” the bartender moved the drink just out of the bard’s reach, earning an offended whine. “You’re not spilling about my personal life to other people, are you?” he accused.

“Wha—me? Pfft, of course not! I was just having a casual conversation with a good friend.”

Diluc flashed a glance to the captain and handed over the drink. “Look, Kaeya, none of this is of your concern, I promise. But don’t read this the wrong way. I have taken no measures to actively hide my relationship. I simply don’t like talking about it. I’d prefer if you’d just respect my privacy, but if you decide to find out about it on your own, I won’t stop you.”

Kaeya practically lit up, sitting straight like a wilting flower finally exposed to the sun. “So you’re telling me you’ll tell me if I guess it right?”

He stalled and chewed on the inside of his cheek. “If it gets you out of my hair, then I suppose…”

A surge of fervor pricked his brain as a grin spread across his face. “Let’s see… Is it Margaret? Marjorie? Sara? Anna? Vi—”

“I’m not going to say anything if you blindly spit out names like this,” Diluc seethed, crossing his arms.

He groaned and splayed himself across the bar top. “Why must you make this so difficult for me… Can I have a single hint? Just one?”

“Ugh… Fine, I’ll answer one question—only if you leave.”

Kaeya huffed in offense, but reluctantly complied with his terms. He racked his brain for something he could use. “Alright… First and foremost, I need to know if this is even worth pursuing, so I must ask—do I know this person?”

“That’s a vague phrase. Like, know personally? If you’re friends with them?”

He shook his head—No, that was too specific. If Diluc crossed that one out, he’d be back to square one. “I just want to know if I know they exist,” he corrected.

The redhead considered the thought for a moment. “Well, yes, you’re aware of their existence.”

“Great.” He thought it over. That only left…

An extremely large pool of people.

He chided himself, realizing the answer was so murky he was at square one regardless. “Wait, I’m ‘aware of their existence’? What does that mean?”

“What, it was the exact answer to your question, was it not? It means you know of them. You may or may not have talked to them, and you may or may not even be close.”

“That’s cryptic as hell.”

Diluc only shrugged in response.

“If it helps, they’re a vision holder as well!” Venti chimed in.

“Venti, I’m going to cut you off if you speak another word,” Diluc threatened. And suddenly, the bard seemed very interested in the sugared rim of his glass.

“Hold on, that question was too vague,” Kaeya corrected. “Could you at least tell me if you’re dating a man or a woman?”

“Why do you want to know?” he shot back, his tone biting.

“Woah, back up. I’m just trying to narrow down my options. Do you genuinely think I care about what kind of person you’re with? Have you seen me?”

Diluc visibly relaxed at this, almost huffing out a laugh. “Right… Well, regardless, I already answered your question. Get out.”

“Eager to see me leave, are we?” Kaeya reluctantly stood from his seat, stealing one final glance at the redhead’s earring. “Alright, but this is hardly the last you’ll see of me. Someone has stolen your heart, so consider this an open investigation,” he declared. Something overwhelming swelled in his chest, almost like a bird hoarding over its nest or a magnet searching for metal. 

He only scoffed as the knight turned to the exit. “Whatever you say, detective.”

Notes:

Man I love the AO3 tagging system but it's kinda funny how the mystery is already revealed to you guys. Dramatic irony I guess? I'm showing this blind to my beta reader though so that they can guess as the story progresses (and so that I can make sure the answer doesn't seem so obvious from the start). Anyways, I wrote this in less than a week thinking it was going to be a one shot, but the intro was so long and already exceeded 4k words so I decided to split it up into a chapter fic. I was SUPPOSED to post it yesterday actually, but got into a little car accident and took a nap so long I actually just woke up the next morning lol. This might be slow to update at first, but I'll get around to it for the sillies <3