Saiko

Saiko used to be on the side of good, but her teammates always seemed to treat her with kid gloves and rarely let her come out and help fight the "bad guys", even though they let her childhood friend go all the time; even though they were the same age and had the same training. It made her feel less like a hero and more like a princess locked in a castle, and she HATED it. She hated having to stay behind at team base. She hated feeling useless. She hated her superiors. She hated her "friend". She hated the world. She hated her life. She hated herself. 

She figured the bad guys wouldn't have any qualms about letting her fight; and maybe she could show her previous team that she could handle herself just fine. So one day she snuck out to join them. At first she was going to dress all cutesy in order to suckerpunch her enemies, but it didn't work, and they seemed to give off the impression that they were holding back, as they didn't want to "hurt" her. They tried to bargain with her to "come back". She was appalled at the sentiment. It was far too late for negotiations. She felt like she wasn't being taken seriously. So she crafted a new persona. One that was much more vicious and definitively "bad". 

She was going to humiliate them. She was going to make them regret having known her. She was going to try to make them feel just as bad as they made her feel; maybe even worse. She might even try to kill her old friend, if she could convince her boss to allow it; he doesn't believe in killing, which Saiko thinks is ridiculous. They're "bad guys", they should be doing bad things, at least in her opinion. Some comic mischief and the occasional tussle just doesn't seem like enough for her; especially with how hesitant her previous team was to let her participate at all. Her new teammates sometimes talk about her as if she's a feral animal. One of them seems actually afraid of her. She likes that.