Flurry of Dancing Flames Axel
In Kingdom Hearts: Chain of Memories, Axel was the trump card for the rest of the Organization as a double agent in Castle Oblivion to keep track of Marluxia's plans. In Kingdom Hearts II, Axel betrayed the Organization for his friendship with Roxas and acted of his own accord to try and help Sora, though he originally wanted to turn Sora into a Heartless to get Roxas back. Axel appears in Kingdom Hearts 358/2 Days as a playable character in Mission Mode and one of the main protagonists.
Lea, his complete being's original name, was revealed in the Day 358's Secret Report and also mentioned by Saïx. Lea was friends with Saïx's original persona, Isa. Lea, Isa, and Axel each appear in Kingdom Hearts Birth by Sleep, Axel's appearance merely a cameo in the secret ending.
He's set to appear in Kingdom Hearts 3D: Dream Drop Distance as Lea, without the facial markings.