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Mirio kisses Tamaki on a summer day when they're 14, and Tamaki loses his breath and his eyes go wide and he kisses back. They'd been inseparable even before then, but now their moments alone, in between classes and internships, in the locker room and on the train home, are full of stolen kisses and knowing touches. It's bliss. No matter what the rest of the world throws at them, being together is bliss.
In their third year at UA, Tamaki realizes that they need to break up. The conclusion had dawned on him slowly, too painful for him to bear, but it's inevitable. He carries the weight around with him for a month, while they bat around thoughts of a hero partnership after graduation. Tamaki can even feel distant hints of marriage in the future, the gears that clearly turn in Mirio's head when someone mentions rings or weddings or publicly-out heroes.
Finally, he brings it up, choking on the words. "Mirio, I love you. More than anything in the world. But I can't live my life knowing that you only knew what being with me was like. Wondering if you only chose me because you didn't know anything else." And it's not enough, he didn't say it as clearly as he needed to, because Mirio rushes to reassure, the way he always does when Tamaki's saying out loud the things his anxiety tells him. But this isn't the same. Tamaki pulls out stories and statistics, people who married their high school sweetheart (middle school sweetheart) and found out about a whole other world of experience far too late, arguing the bitter facts until Mirio understands that this isn't just something he needs comfort about.
Mirio's expression is unreadable. "Fine, then let's date other people," he says. "If you want me to date other people, then we can date other people. We don't have to break up to do that, it's a thing." He looks defensive, like he's ready to argue, but ... that makes sense. Perhaps their heads will be clearer, even, without being distracted by the pain of being away from each other.
Mirio starts dating Nejire soon after graduation, which isn't that surprising. They've always had chemistry, even if their long-term compatibility remains to be seen. Tamaki does his best to be open to new relationships, though he doesn't expect to find anyone soon — or maybe ever. It's hard for him to imagine feeling anything like what he feels for Mirio for someone else. Even someone whose company he prefers to solitude is hard to find.
But his kouhai is different. Fatgum's agency is a pleasant enough place to work that Tamaki sticks with it even after graduation — and if he's honest, most of that can probably be attributed to Kirishima-kun. He's as bright as the sun (like Mirio), but he's kind and understanding (like Mirio). He can't count the number of times Kirishima has cheerfully stepped in to save him from a nosy reporter or an overeager fan. More and more, they find themselves simply spending time together outside of work, seeing the sights of Osaka, finding new places to eat, or simply hanging out together (mostly at Tamaki's place). Slowly, Tamaki lets himself fall.
To work up the courage to ask Kirishima out, he turns to Mirio, of course. At first, he'd assumed it'd be hard to talk to each other about their other romantic interests, but the jealousy Tamaki was expecting to feel towards Nejire never materialized. Once he made peace with the fact that Mirio would only leave him for Nejire if Mirio in fact chose to do so, any jealousy simply faded into his background fears of inadequacy, which he'd become more or less able to ignore after four years together. And it's sweet when Mirio gushes over Nejire's cuteness.
Mirio reacts to the subject of Kirishima with an easy curiosity. He initially even offers to tell Kirishima himself that Tamaki's interested, and Tamaki has to explain through his hands that that would be even more embarrassing. When Mirio brings up the fact that Tamaki should float the idea of polyamorous relationships first, Tamaki nearly chickens out altogether. It seems too absurd a possibility, to think that Kirishima would be both interested in Tamaki and okay with their relationship style. Mirio stays up late into the night talking through it with Tamaki, and come morning, Tamaki's only half sure he'll ever be able to go through with it.
"Kirishima-kun," he begins hesitantly one day, when the two of them are sprawled half on top of each other on Tamaki's couch, watching TV. He's so much more physically comfortable with Kirishima than with anyone else but Mirio, but he knows it doesn't mean the same thing to Kirishima. He can't stop himself from shaking. Mirio kissed him first — he's never had to do this. But Kirishima won't even think of Tamaki as a potential date until he explains his and Mirio's relationship. He takes another deep breath.
"Yeah?" says Kirishima, sitting up a little as he notices Tamaki's discomfort. He's so perceptive, so attentive to Tamaki's distress.
"W-what do you think of people who date more than one person at the same time?"
"Like ... without telling them?" Kirishima says cautiously. He's facing Tamaki now, his arm across the back of the couch.
Tamaki shakes his head. "No, I mean, where everyone knows who everyone's dating. Could you ever see yourself in a relationship like that?"
Kirishima drops his head onto his shoulder as he thinks. "Well, I guess, sure. I don't know how that would work out, but I'd be willing to give it a shot." His cheeks pink as he asks, "Are you asking because ... uh, you know, is that the kind of relationship you're in?"
Tamaki nods, looking down at his lap, the tips of his ears red. Maybe it is possible?
Kirishima inches closer, his arm coming to rest on Tamaki's shoulder. He looks like he's about to say something, but then he doesn't. He touches his forehead to Tamaki's instead.
"Mirio knows that ... that I wanted to ask you out today," Tamaki stutters out, and Kirishima's reaction is instantaneous.
He breaks out into a broad grin and pulls Tamaki close with both arms. "Tamaki! I love you!" he blurts out. He nearly kisses Tamaki before turning it into a shy kiss on the cheek, but he still looks beside himself with excitement.
When they'd started this experiment, this is what Tamaki had hoped for: Mirio would get a taste of the rest of the world, date a myriad of different people, and in the end decide that Tamaki truly was the one for him. They'd get married and be together as a happy couple for the rest of their lives. But Tamaki is starting to think that isn't the best possible ending.
He's only been dating Eijirou for a few months, but he feels like he never wants to be without him. To Tamaki's relief, Mirio and Eijirou have become fast friends, and the three of them spend time together often. Mirio and Nejire break up, with no hard feelings, when she decides she likes girls and monogamy, but when Tamaki and Mirio talk about moving in together, the question looms in Tamaki's mind whether Eijirou will join them — then, or ever.
After a certain amount of back and forth, they decide ... they'll leave space to grow. They settle on an apartment with one more room than they really need — it's not like they can't afford it on the salary of two pro heroes. Eijirou's in his last year of high school, now, and Tamaki doesn't want to pressure him to join the two of them in Osaka, but it's hard to hide how bad he wants the three of them all to live together.
He and Mirio attend Eijirou's graduation together. They don't get to spend as much time with Eijirou on the day of as Tamaki had hoped, since his family showers him with so much attention. So the next day, both Tamaki and Mirio take the day off work, and invite Eijirou over to celebrate. They're all in high spirits, and get more than a little tipsy.
As they cuddle in Tamaki's bed together, in between kisses, Eijirou whispers, "How would you feel about me moving in with you guys?"
And as he squeezes Eijirou in excitement, and is squeezed from both sides in return, Tamaki answers, "That would be amazing."