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

[img]http://24.media.tumblr.com/3b280fafb72cad0f718be64368e64ca7/tumblr_myylzmuiY41rw3pzao4_250.gif[/img]
Like educational manga? take a look at Ed U. Manga's list of manga here