Chara change is when your chara switches your personality to who you want to be.
It may seem embarrassing (trust me, it is) but the chara isant trying to embarrass you. She\he is trying to show that that is what you would be like when you are your would be self.
Charas do it either just randomly, or when you most need it.
And you can chara change when you need to fight X eggs and can't transform. (I'll mention tomorrow)
