D. Gray-man

"D.Gray-man is the story of Allen Walker, who roams a fictional 19th century Earth in search of Innocence, a mysterious substance used to fight demons called akuma. Allen questions whether he is willing fight the Clan of Noah, followers of the Millennium Earl. After all, he signed up to kill akuma, not humans! With his left eye injured, he can no longer tell akuma from humans, making him a target for attack. Despite this weakness, he is sent by the Black Ministry to find and protect his master, General Cross."