Kenichi Kokuho became lost in the mountains as a small child. A foster father raised Kenichi; when Kenichi was 15 the foster father told Kenichi about his background and the teenager returned to the civilized world at age 15.

(Source: Wikipedia)

Included one-shots:

Volume 17: Moeru! Oniisan (Pilot), Boku no Tomodachi P.2

Volume 18: Super Otousan