Parasyte -the maxim-

Migi was meant to crawl into Izumi Shinichi's ear while he slept and replace his brain with itself. Migi got lost, settled in his right hand instead, and now the two of them are stuck cohabiting — a high schooler with a small alien parasite where his fingers used to be, watching news reports about people being slaughtered and rearranged by other parasites that succeeded at the takeover. Migi sees no problem with the killings; Shinichi does. The series is their argument, the body count, and what is left of either of them after they spend a year learning to live in the same hand.