Takumi had always been the pushed into the shadows a bit. He'd never stood out the way that Alaska and the Colonel did. You could even tell from their names that they were important and empowered – he just wasn't like that.
He blended into things, with his eager-to-please attitude and unobtrusive existence. He settled for being Alaska's go-to guy and the Colonel's back up man, because really, what choice did he have? Takumi was not the type to make a fuss or drama; he was not the type to make them think of him as more.
Needless to say, he felt shafted when Pudge came along. Sure, Pudge was great and everything, but he was a new kid. A new kid, and they still liked him better. Well, that's what it felt like, anyway. He could get along with the Colonel, the way that Takumi never could. He dropped Takumi in importance almost immediately.
Alaska, she did not forget about him. He was relieved about this, if nothing else. But it became all the more clear what his role in her life was: her best friend. And not the type that developed into something more, that would have been Pudge. Alaska, for all her intelligence, she was a bit oblivious to him. She could not, could not, have known about Takumi's feelings for her, or she would've been gentler with him. Or at least that's what he tells himself.
He loved her, more than anyone else in the world. He loved her before Pudge, before anyone else did. He'd loved her first, and at least he can tell himself that. She may not have loved him back, but at least she was his before she was Pudge's. But she was never his. And this is why he keeps the secret, why he does not tell them of his mistake.
They can keep their explosive, theatrical last nights with Alaska, because he has his secret. It is what he does best after all, staying quiet. He sticks to the walls, keeping anything dramatic under wraps – it is the only way he can remain marginally in power.
In the end, he tells them. And he leaves. It was the inevitable ending to such a disaster; he couldn't have kept it to himself forever. Now that Alaska's gone, he really doesn't need to stay behind. The Colonel and Pudge don't need him – they'll probably be better of without him around, haunting them. He knows that, and it hurts him, and makes him resent them.
As he's on the plane, flying home, he wonders what he really liked about them in the first place. He can't remember.