Hmm, I'd say Sensei is probably an average to above average height and dark curly hair guy.  17-early 20s?  He likes hentai, or maybe just anime girls in hentai poses?  For some reason, I relate that to a thin guy.  Though, I see stereotypes of only fat kids in their late 20s in their parents' bedrooms as the only people who like hentai, but that isn't the case with people I know who like it, so I am going to go that route with you, and assume you are a bit thinner.