Kingdom Hearts 3: 10 Final Fantasy Characters Who Must Appear
4. Leon
Leon has featured quite heavily in the storylines of the first and second Kingdom Hearts games. Even though his popularity can't quite match up to Cloud, he's definitely one of the most important one in the Kingdom Hearts universe.
Appearing as an interpretation of Final Fantasy VIII's protagonist Squall, Leon has acted like a mentor and bigger brother to Sora in both numbered Kingdom Hearts titles.
Kingdom Hearts 3 seems to be big on the idea of growth and becoming more powerful, which will hopefully be shown with the difference between Sora and Leon compared to their first meeting.
The best way to implement Leon in Kingdom Hearts 3 would be to have him and the rest of the Hollow Bastion Restoration Committee catch up with Sora and the gang on his adventure.
If there's another big battle like there was in Kingdom Hearts 2, it'd be great to see Sora and Leon team up one more time and truly solidify their status as mentor and student.