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"I’m going to marry you, Kakashi! That’s a promise!" Gai had declared one day after losing one of their challenges, complete with his nice guy pose and everything.
Kakashi rolled his eyes. Just another stupid declaration from Gai's mouth. "You can’t marry me, Gai," he said, as patronizingly as a seven year old can muster. "We are too young according to Konoha law."
Gai laughed. "Of course not now, Kakashi! One day when I am worthy enough to call myself your equal, that will be the day when I marry you! Just you wait, we will be betrothed!"
This one exchange between children in the woods came and went, but it always stuck out in Kakashi's mind. Gai had spoken it with such conviction, there's no way he didn't mean it. It made something twist in Kakashi’s chest every time he thought about it. His mind kept snagging on that one word: betrothed.
But then Kakashi lost his father, and then Gai lost his father, and they were on two different pathways. They grew up, grew apart, grew back together, grew into their skin and out of their ideas about the world.
But through it all, their rivalry remained. They pushed each other to be better - faster, stronger.
When the day came that Gai finally won a challenge against Kakashi, he thought "okay, here we go," and braced himself for Gai's insistence that now they get married. But to his surprise, Gai had simply said "Excellent spar, rival!" with a modest amount of sparkles, then suggested they go eat a bowl of curry to "rejuvenate their expelled youth." Kakashi spent half the meal trying to determine why he didn't say it was time to get married, and then spent the other half trying to determine how he felt about it.
Challenges came and went, and their rival score became closer and closer. Gai might be winning more challenges, but he was still behind in their overall score. So then Kakashi figured he was waiting until they were even - he did say equals after all - but on the day Gai finally surpassed him, he still didn’t mention it.
Then suddenly, in the blink of an eye, they became senseis to their own group of genin. Gai was busy with his team, and Kakashi with his, and their challenges looked less like no-holds spars and more like sushi eating contests, the real reason behind them to catch up and enjoy each other’s company. Kakashi liked those best, when it was just him and Gai and a mountain of empty sushi platters. Those moments it was easy to look into Gai’s eyes and remember that they were betrothed, in a way.
Suddenly, their peace left them just as quickly as it came on, and they were in the middle of a war. Suddenly Kakashi was watching Gai as he tore open his body and launched himself at the enemy with a smirk. Suddenly Kakashi’s heart was in his throat as he watched Gai do what he had prepared his entire life to do.
But we were supposed to get married, Kakashi’s mind helplessly supplied. It was the only thing he could think of as he watched Gai's body get ripped apart by the power contained within.
Gai had said it was a promise, and Gai never made a promise he didn’t keep. He promised he would marry Kakashi, he promised their lives would be together forever, and Kakashi’s heart broke in two as he watched Gai break the only promise that ever really mattered to him.
By some random miracle, Naruto had saved Gai. Half-hearted jutsu in a semi-formed chakra bond that was cast in a distracted moment meant the difference between life and death, for both Gai and Kakashi. There was no reason it should've worked. Even Naruto himself couldn’t explain it.
Yet, when Gai's eyes finally opened after months of being comatose, Kakashi couldn't help but sob out his relief.
Weeks of continuous intravenous medication, months of physical therapy, Gai's road to recovery would be a long one. Tsunade kept reminding Gai not to overdo it on the training, and Gai sheepishly promised he wouldn't as he hid the weights Rock Lee had smuggled into the hospital for him.
Eventually Gai was discharged. Kakashi thought surely now that they almost died and lost everything that mattered to them but had survived despite it all that they would get married, right?
But still, Gai said nothing.
Gai's recovery happened alongside Kakashi's appointment to the Hokage's office. Kakashi wanted to spend every free moment he had at Gai’s side, helping him in his recovery, but Gai had insisted he commit himself fully to the new role he was appointed to. The citizens needed him, Gai kept reminding him.
But I need you, Kakashi’s heart kept reminding him.
At Kakashi’s insistence, they moved into the Hokage’s residence together. It was all one floor, easy to maneuver a wheelchair through, and it was much too large for just one person anyways. Or, at least, that’s what Kakashi said when he convinced Gai to move in with him.
A couple years passed by, and Kakashi watched Gai get stronger and stronger.
And he waited. And still, Gai said nothing.
They were sitting on their back deck in their garden, watching the sun go down behind the hills, the world painted in pink and purple. Kakashi looked over at Gai, the way the evening sky reflected off his hair, the twinkle of the remnants of sunlight reflecting in his eyes, and suddenly he couldn't hold it in anymore.
"How about now?" he asked softly, and Gai turned to look at him.
"Hmm?" he questioned, tilting his head slightly.
"Are we equals now?" Kakashi asked, and the look of recognition on Gai's face told him that he knew what Kakashi was talking about. What Kakashi wasn't expecting was for his shoulders to slump, and that look of recognition to turn into a look of sadness. Gai looked back over the hills.
“I didn’t think you remembered that,” he murmured.
“Of course I remembered,” Kakashi said. “I think I’ve been patient long enough.”
“I said when I could call myself your equal. You’re the Hokage,” Gai said, looking at the hands in his lap. “I can’t ever catch up with you, rival. I can’t ever be your equal.”
Kakashi sat up. "So are we just going to be engaged forever? You're never going to make an honest man out of me?"
"Don't make fun of me, Kakashi," Gai whispered. "Not about this. I've loved you for too long to be able to laugh about this. I can’t make good on my promise to you, so I’ve committed myself to being by your side in support. It’s my self rule," he said sadly, looking over at Kakashi like he was everything he wanted, but couldn't have.
A strong surge of fondness threatened to overtake Kakashi’s entire body, so he reached over to turn Gai's face towards him with two fingers under his chin. "You tore your body apart in order to protect this village. You saw something precious enough to give your life for, and were prepared to pay the ultimate price for it."
"You died, too," Gai grumbled, and Kakashi couldn't help but laugh a little at that.
"Only you would be competitive about dying,” he said with a fond shake of his head. “My point is, you’ve pushed us to better ourselves for our entire lives. You sacrificed for this village just as much as I have. I would say that makes us equals. The question is, do you?”
Gai looked at him skeptically. "You want to marry me?"
Kakashi shrugged. "We've been engaged since we were kids. I think it’s about time we tied the knot."
The answering smile on Gai’s face made their decades long engagement worth it.
“Kakashi, my beloved rival, may I kiss you?” Gai asked, eyes shining.
Kakashi responded by lowering his mask down and leaning in, a soft brush of their lips more electrifying than any chidori Kakashi could muster.
They were married the next day. Kakashi refused to wait any longer.