Theory: I think Aryka is Yoen's brother, and that Aryka was also part of the accident. So now he's dead or in a coma for a very long time. If he's still in a coma, I think that would explain why he still has his human feelings, making him diff from others. Coz others are already dead, while he's still fighting for his life??
I dunno, I just dwelled too much on the difference between Aryka and Yoen as it is often brought up in the story. Plus when Yoen said that he and his brother are very opposite. I just thought that maybe it's foreshadowing or something like that. (〜 ̄△ ̄)〜
OMGGG MY THEORY IS CORRECT!! THEY'RE BROTHERS!!