Seth is played by the tubby, hilarious Jonah Hill. That's pretty much all you need to know about this Superbad character. He's focused on tapping some serious booty for most of the movie, only to realize how important friendship is by the end.
Seth: [referring to Evan's mother] I am truly jealous you got to suck on those tits when you were a baby. Evan: Yeah, well, at least you got to suck on your dad's dick. Seth: [shrugs] It's a nice lookin' dick.