For those who think that only Cloud and the Keyblader can see Tifa, I suggest you replay the game, specifically, the part when Tifa aenters the secret room where Ansem's computer is. Both Leon and Tifa are in the room at the same time. If you talk to Tifa, she congratulates you on finding the King, and says that "I guess that if you keep looking, you'll eventually find your man". If you talk to Leon, he makes a comment about Tifa. Can't remember what it is, right now, but it is definitely there.