Note; This is an opinion, do not take this as anything offensive or insulting. This is to help you avoid having an overpowered or overangsty character.
It is very fun to make your own character, or fan character, yes? ^^ I agree. But, sometimes, people don't go through character development the correct way, having their character or fan character result in being overpowered and/or undefeatable.
Sometimes, they do follow the right steps, but instead create a much more angsty, annoying, depressed character.
I will start off with the much more common character; Mary-Sues and Gary-Stus.