Deathstroke (Slade Wilson)

Season One
Deathstroke was a mercenary working for Ra's al Ghul. He trained Bruce Wayne and Talia al Ghul. In The Stroke of Death, he goes to Gotham to kill Bruce as per Carmine Falcone's instructions. It is later revealed, however, that Slade didn't come to kill Bruce, but came to bring him back to Ra's. When Bruce refuses, Slade reveals that Ra's didn't send him. It is revealed that Slade's compatriot Ubu knew how important Bruce was to Talia, and wished to convince him to come back to her. When Bruce refused again, Ubu told Slade that their work was done. Slade's response however, seemed to signify that he disliked failing.