Twincest, well sort of. I just loved this so much!! (What can i say, i'm just twisted like that)
Update: i came back to reread this after years and it's still so beautiful and so fucking angsty. I finally understand what kei meant by some hole that needs to be filled, it's like his heart (or half his heart) was with Roku and he has been searching for that missing piece all his life and I'm just gonna start sobbing
Sora o Daite Oyasumi