Seth Green played ubercool guitarist “Oz” Osbourne, who became Willow’s first (and only) boyfriend. Mellow, with a spectacular dry wit, Oz became an important part of Buffy’s demon-fighting team (even though he was a werewolf himself). It was only when Oz’s inner wolf compelled him to cheat on Willow that things went south and he left Sunnydale to tame the demon within.