He was a hero who "debuted" just after Zero Hour, where he was revealed to have been a founding member of the JLA, but in the course of their first mission, he saved the world, but caused a sort of hiccup in the timestream, which removed him from the memories of anyone who had known him. When he returned all manner of things happened, some interesting, some... not.

More details here (http://en.wikipedia.org/wiki/Triumph_(comics))