Kingdom Hearts Help? Roxas, Axel, Riku, and Xion?

Um which of these games has Xion in it? I think in the game, Roxas, Axel, and Xion are friends, and Xion and Riku are friends. Is the game Kingdom Hearts Birth By Sleep (KH: BBS) or Kingdom Hearts 358/2 Days (KH: 358/2)? Or maybe is it another game altogether? I'm so confused...ugh