Responses

Its not Kei who lived because Kei was already dead inside to the point where you could see it in his eyes, he would not be able to function if it were Roku who died, he'd most likely kill himself again.
This is Roku, going through PTSD and a part of him is still searching for answers and he's probably punishing himself by doing the things his bro did.
Also, plot twist. What if the one who fell and accidentally died was Roku and Kei is living and pretending to be Roku?