Some weird side stages open up later in the game, TVC - each of them are unusually designed and have strange goals requiring abilities that only certain characters have (like Colet's being able to create bridges and ladders).
I hated that stupid frog too, Arde0. That was just insulting. The fact that later on...
spoiler (click to show/hide)
...it actually becomes a playable character made me go WTF. The fact that it can use one of the ARMLETS (!!!) was just absolutely distinguished mentally-challenged.