Johan Liebert

He is a ruthless serial killer and psychopath whose life was saved by Kenzo Tenma as a child. He is also the twin brother of Nina Fortner. Johan has stated that his true goal is to be the last one alive when the world ends - but upon rediscovering a children's book that seemed to play a key role in his past, he decided to commit the "perfect suicide". i.e., dying without leaving behind any evidence of his existence (with the possible exception of Dr. Tenma's memories of him, in order to destroy Tenma's faith in humanity).