[Okay, now it's all about saying just enough to get Sokka to want to go talk to Superman without saying more than he should know. Good thing Sokka probably has no idea how much Clark should or shouldn't know about Earth supervillains.]

He calls himself Lobo, or "The Main Man". He's an alien and a supervillain, and he's got some incredible powers, though I guess you know that last part already. If you want to know more it'd be best to talk to the League, since they're the ones who usually deal with him when he shows up. Especially Superman.