I know who "Tomotachi is"

Salón del Manga 2007 007, originally uploaded by Barutaroku.

If you dont wanna know please scroll away now,
here goes, Tomotachi a Guy that's not part of their childhood
he is someone that lives far away, but met Kana's Mother and so
they got together, later this guy found kenji's book very interesting
and he was jealous of such a beautiful childhood they had.
so he wanted to be part of it. with such ambition, he made the fiction in to reality.

so there you go the secret of Friends "tomotachi"