A mangaka whose works are all male-oriented manga, particularly in the shonen demographic. He falls in love with Sono Hanasaki, a mangaka and photographer who writes pornographic (hentai) manga. Seiji Yamada first meets Sono-san after he finds Sono Hanasaki's lost camera, but is surprised to find out that Sono is a guy! Even though Sono-san is a man, his ridiculously pretty face and erotic gestures seduce Seiji anyway.

It is mentioned Seiji Yamada is inspired by Hanasaki-sensei's drawings of beautiful, erotic bishoujo in his manga.

He is voiced by the singer of CONNECT, Kenichi Suzumura.