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Even though he isn't in every scene of the book, Takumi plays a pretty important role in Miles's life and in Alaska's death, though we don't find out about his participation in letting her go until the end.
Takumi serves as a sort of conscience for Miles somewhat regularly, especially when Miles finds out about Alaska snitching on her roommate and when Miles ignores Lara for, like, two months (46after.3). But other than this, Takumi is left a kind of two-dimensional character, and besides his mad rapping skills, we don't really know much about him besides the things that come up in conversation within earshot of Miles. We know his grandmother died, and we know he's not a virgin, but that's not enough to make him really pop off the page.
We start to get more of a picture and less of a sketch of Takumi after Miles and the Colonel stop being jerks and cutting him out of their lives. Takumi is understandably annoyed by their standoffish actions and secretive behavior:
"I'm tired of you acting like you were the only guy who ever wanted her. Like you had some monopoly on liking her." (29after.9)
And he's got a point. Miles and the Colonel have been so consumed with their own guilt and own loss that they have ignored their other friends, effectively pushing them away and failing to recognize that there are other people struggling around them too.
Takumi is really good at keeping secrets. And at investigating stuff. Not only does he figure out that it was Alaska who ratted out Marya before Alaska tells him (67before.15), but once he does, he doesn't even tell the Colonel, one of his closest friends.
This all leads us up to Takumi's big reveal at the end of the novel, when he admits to Miles and the Colonel that he, too, is responsible for Alaska's death:
For a long time, I was mad at you. The way you cut me out of everything hurt me, and so I kept what I knew to myself. But then even after I wasn't mad anymore, I still didn't say anything, and I don't even really know why […] I had this secret. (136after.4)
Just like Miles and the Colonel, Takumi saw Alaska the night of her death and did nothing to stop her from driving away. So he shares the culpability that Miles and the Colonel do, though he deals with it mostly off-screen. Because we're so hung up on Miles/the Colonel, we never really get to know how Takumi copes with her death, and instead only get little glimpse into his psyche in the letter he writes to Miles/the Colonel:
I don't know what I was thinking. So I let her go, too. And I'm sorry. I know you loved her. It was hard not to. (136after.6-7)
We know, Takumi.