(Better description in later update)! Roxas & Axel were best friends whilst in the Organization. Sora couldn't really make head or tails of Axel when he met up with him in Castle Oblivion. A year later, Axel tried his upmost hardest to reminds Roxas about him and the rest of Organization XIII. After failing on several attempts, Axel decides to fight him. After losing, Axel created a plan. Roxas had 'joined' back up with Sora, with Sora unawares. Later on, Axel meets up with Sora, and tries to get Sora to help him out. Later, before Axel is 'destroyed', he tells Sora about Roxas. When Roxas meets up with Sora later, he seems happy to have him as his 'other half'.
Without Sora, Roxas couldn't exist, and vice versa. At the same time, Axel proved to be loyal friend, ovedrall to both of them. Had things have been different, perhaps this would have been the key trio of the game, even.