Chapter Text
The first time Naomi noticed it, she thought she was imagining things.
The faint click of the lock turning at midnight. The soft scuff of footsteps on the wooden floor. The way her brother’s bed remained suspiciously neat when she peeked in the next morning, like no one had actually slept there.
At first, she brushed it off. Maybe Junnii was just working late again — the Agency wasn’t exactly known for its normal hours. But then came the second night, and the third, and the night after that when she swore she saw a shadow slip past the hallway, followed by the distinct sound of Tanizaki’s door closing and… locking.
Naomi was many things — dramatic, devoted, nosy — but above all, she knew her brother. And her brother never locked his door unless he was hiding something.
So, like any good sister, she did what any rational, perfectly mature adult woman would do.
She planned a full-scale surveillance operation.
Day 1: Naomi’s Suspicion
“Yosano-sensei,” Naomi whispered, crouched beside the doctor’s desk with a conspiratorial gleam in her eyes.
Yosano didn’t look up from her paperwork. “Naomi, it’s ten in the morning. If this is about Tanizaki forgetting your birthday again, I’m not hiding his body.”
Naomi pouted. “No, no! It’s serious! He’s sneaking out at night — or worse, someone’s sneaking in!”
That earned Yosano’s attention. She lifted her head, brows arching. “Someone? Into his room?”
Naomi nodded gravely. “Every night around midnight. He locks the door. He’s acting weird. I think…” she leaned forward dramatically, voice dropping to a whisper, “...he’s hiding a lover.”
That was when Kunikida walked by, overhearing just enough to pause mid-step. “Tanizaki? A lover? Impossible. He spends too much time with Naomi for that.”
Naomi gasped, affronted. “What’s that supposed to mean?”
“Exactly what it sounds like,” Kunikida replied, pushing his glasses up with precision. “Still, if he’s breaking Agency curfew, that’s grounds for discipline. We’ll have to investigate.”
Within minutes, the conversation had snowballed. Yosano, intrigued. Kunikida, indignant. Atsushi, confused. Kenji, excited. And Dazai — Dazai, of course — delighted.
“Ohhh, a secret romance under the stars?” Dazai swooned theatrically, already scribbling notes. “I vote we form an investigation team. Operation: Midnight Secret!”
“Stop naming everything,” Kunikida snapped.
Day 2: Stakeout
That night, the Agency assembled at Naomi and Tanizaki’s shared apartment.
They had divided tasks:
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Yosano handled the recording equipment (“For scientific purposes,” she claimed).
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Kunikida brought a notebook and stopwatch.
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Atsushi was stationed by the door to relay sound.
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Kenji had snacks.
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Dazai had binoculars and no self-control.
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Naomi, of course, was the self-appointed commander.
“Remember,” Naomi whispered dramatically from her hiding spot in the hallway closet, “no one makes a sound. We wait until he sneaks out — or whoever sneaks in — then we strike!”
Kunikida sighed, writing Naomi’s irrational behavior in his notebook for future reprimands.
At exactly 11:58 PM, the group froze. A faint shuffle came from Tanizaki’s room. Then— click —the door locked.
Naomi’s eyes widened. “He did it! He did it again!”
Through the narrow slit of the door, they could hear Tanizaki whispering. Softly. Hesitant. And… giggling?
“Oh my god,” Naomi mouthed, hand over her mouth. “He’s not alone!”
Dazai grinned. “Scandalous~. Who knew our dear Junnii had it in him?”
“Quiet!” Kunikida hissed. “We need proof before we—”
Then came another sound. The muffled slide of a chair. Footsteps. A thump. A rustle of cloth.
Naomi nearly fainted. “That’s… that’s a bed sound!”
Yosano smirked, crossing her arms. “Well, well, well. I didn’t think he had the guts.”
“Maybe we should knock,” Atsushi whispered, blushing furiously. “I—I don’t think we should be listening to this.”
But before anyone could stop her, Naomi slammed open the door.
“JUNNII!!! WHO IS SHE?!”
The group burst in after her — Dazai tripping over Kunikida, Atsushi covering his face, Yosano grinning like it was Christmas.
And there, in the middle of the room, stood Tanizaki Junichirou — completely clothed, wide-eyed, and looking like a deer in headlights.
He wasn’t alone, though.
Sitting beside him on the floor, surrounded by a half-dismantled projector and blueprints, was Tachihara Michizou.
The Reveal
“Tachihara?!” Kunikida exclaimed. “What are you doing here?!”
Naomi froze mid-accusation, her dramatic pose faltering. “Wait. You’re— You’re with him?”
Tanizaki blinked rapidly, face flushing bright red. “N-No! It’s not what it looks like!”
“Oh, so it’s not a romantic rendezvous in the dead of night?” Dazai said cheerfully, leaning against the wall. “Because it really looks like one.”
Tachihara sighed, rubbing the back of his neck. “I told you this was gonna blow up, Tanizaki.”
Yosano arched a brow. “You two do make an interesting pair.”
Tanizaki threw up his hands. “No, no, no! We’re working on a joint investigation!”
“Investigation?” Kunikida repeated skeptically.
“Yes!” Tanizaki said, standing up hurriedly. “We found a pattern in the Armed Detective Agency and Hunting Dogs’ joint mission reports — data inconsistencies, missing files. We’ve been comparing information to find the leak.”
Naomi blinked. “So… no secret girlfriend?”
Tachihara looked vaguely uncomfortable. “Unless you count him as one.”
“HEY!”
Yosano chuckled. “So, this was all… work?”
“Yes,” Tanizaki groaned, dragging a hand down his face. “And I locked the door because I didn’t want you thinking exactly this!”
Naomi crossed her arms, unimpressed. “Well, maybe if you told me you were sneaking in spies after midnight, I wouldn’t assume you were dating one.”
“‘Spies’?!” Tachihara exclaimed, exasperated. “I’m a police officer!”
“Formerly!” Dazai chimed in, ever unhelpful.
Kunikida pinched the bridge of his nose. “So let me get this straight. Naomi drags half the Agency here because she thought her brother had a lover, and in reality, he’s been secretly collaborating on a classified investigation?”
“Yes,” Tanizaki muttered.
Yosano smirked. “Adorable.”
Naomi pouted. “You could’ve just told me, Junnii…”
Tanizaki sighed, finally softening. “I didn’t want to worry you.”
A pause. Then Naomi sniffled. “I thought you replaced me…”
The room fell silent. Atsushi looked guilty. Kenji offered her a cookie. Dazai theatrically dabbed his eyes with a tissue.
Tanizaki sighed, stepping forward to pat her head gently. “You’re irreplaceable, Naomi. Now please stop breaking into my room.”
Epilogue: The Aftermath
By morning, word had spread through the Agency.
Dazai had already sent an email titled “Tanizaki’s Secret Midnight Rendezvous EXPOSED”, complete with doodles.
Kunikida spent the day updating his Code of Conduct to include ‘Unauthorized sibling surveillance prohibited.’
Naomi sulked for exactly two hours before declaring herself “Head of Internal Affairs,” and setting up more cameras in the apartment “just in case.”
And Tanizaki? He spent the rest of the week being teased mercilessly by everyone — even Kyouka — while Tachihara refused to show his face at the Agency for three days straight.
Still, when night fell again and Tanizaki sat by his desk, typing in the quiet, he couldn’t help but smile faintly. From outside, he heard Naomi whispering something to Atsushi about “keeping an eye out tonight.”
He sighed, locking the door once more.
Because if his sister thought this was suspicious… she’d really lose it if she knew Dazai was helping next time.