Tips if You want to unlock all character:
- search for levels starting with falling monster. One will grant access to special characters (on the bottom of choose menu) and second to to big ones (4x4 4x2). I think it's hardest to collect these two groups. Rest You could make, by the way.
 "Mitoza"){kind=icon}
 "Great Dungeon in the Sky"){kind=icon}
 "Little Protectors"){kind=icon}
 "Balloon in a Wasteland"){kind=icon}
 "The Company of Myself"){kind=icon}