@Twilight: lol? Isn't that like... the 2nd disc? Start playing again! Leon demands it!
No, the letter can't be from Mickey... doesn't something pink fly out of it? Mickey... pink... who knew?
And yea, he does know Leon. During the "War" on Radiant Gardens, when Sora & Co. were going to help out, Mickey shows up and tells them to leave the world. He tells them that "Leon and the others can take care of things here.", or something of the sort..