Black Haze maybe? he definitely does have a hidden past which you find out right away when you read chapter 0, but so far they have never really mentioned it again yet.

Natsume Yuujinchou - he hides his past because he was bullied a lot from being able to see spirits.

Dalsez -though in this one its not that he forgot his past its that he was too young to remember what happened



________________
__________________________

Like educational manga? take a look at Ed U. Manga's list of manga here