:/ ok so... I now have Crystal Mickey! Right now, he's level 6, so I haven't quite finished with leveling him... but the 100-star reward has been revealed! (Updated my main post on page 3 with those details.) I still can't figure out what that final unknown character in the progress screen is, though, and now that I have 100 stars, I really don't know how to unlock it!
The only thing I can possibly think of that I haven't done yet is scan all the ROUND power discs in to the game. Would that possible unlock the final character? I had only been using User Control/Mickey's Hat to level up my characters more quickly. Maybe I should compile a list of exactly what benefits each disc has?