I remeber reading a manga a while ago about a kid with another person in him. I think it was seinen and the evil half looked like the guy from soul eater.

I don't remeber it well so the above description may not be right.

Can anyone help me out?

Split personality? Is it like the whole main plot is about his split personality or it just sometimes come out? Cause if it's a just a support point [like only 1 or 2 chapters his split personality comes out or something].. there is a lot of manga like that... But if it's like the whole series is about it.. then that would narrows it down o.O

R-18 below
[img]https://dl.dropboxusercontent.com/u/32718267/aot.jpg[/img]
Detective Horror Series

Tracker BakaBT

Kurozakuro?

try 666 satan, otherwise known as O-parts hunter. Its about a kid with the devil lucifer inside him and was created by KISHIMOTO Seishi

Fairy Cube maybe? But it's not a seinen at all.

What I like in Trivial Pursuit style. Pick your category:[img]http://i379.photobucket.com/albums/oo233/Reid4891/Manga/trivia-1.jpg[/img]

it was Kurozakuro thanks. guess it was shonen.