Lexaeus is not very known. He appears in Chain of Memories Re: Reverse Rebirth, the Riku story you activate after you beat Sora's. He is really powerful, and his mission is to eliminate Riku, he tries to create a copy of him, but Riku's darkness defeats him. He, or better, his somebody, Elaeus, worked with Ansem in BBS, in Radiant Gardens.
He wields a giant Tomahawk, and is the most powerful member. He' a playable character in KH 358/2 Days, and he's really strong. He also has a weird face, with something like a permanent disgust expression or something like that... I don't really like him, he's ratherly prepotent.