I now nearly completely believe Lea to be the missing link. Before I wasn't sure due to the circumstances mentioned in the first post. But since Lea now has a Keyblade and is strictly involved with both Sora and Riku (and Kairi, as things are hinting at), he is the 4th friend in the square of friendship. Also, since he isn't a Nobody anymore, he fits in almost perfectly. There is now only one thing left that still makes me think twice about it, and that's the thing with his name, also mentioned in the first post. Lea doesn't mean anything of fiery nature as far as I know. Least of all in latin. In common, I'm not completely sure why he was given this name to begin with. I don't think that his name DOESN'T fit him, but I'm curious about the meanings it has that Nomura finds to pertain to Lea in whatever context during the creation of his character. I bet there is something I haven't picked up on. Something clever that hasn't been explained. Or that I didn't see o_O.
Aaaanyway, yeah, I'm thinking Lea is the 4th friend of fire (catchy, no?)