from what i saw some boss transform depending on who you pick
mc=first boss, the girl with twin tails/where you learn to jump=brown hair guy, swimming girl= tan guy, green hair girl= blond guy under the rope where the bike is, the pink hair girl=??? tried all the characters that were alive the guy with brown outfit and guy with blue outfit non make her transform also looks like im missing 2 characters when you die the final 2 on the right,
and looks like there also a gallery but from what i can see im missing 2 bosses and 2 secret bosses one im missing is the one in green hair girl floor i could not beat and im pretty sure the last one is in pink hair girl floor there a door i could not reach with a candle that not lit