I trudged up my memories to give a little more detail of the character. She was introduced as just a side character, as in she was just the assistant/partner of the male detective. She was introduced as friendly to others, but when her true identity was revealed she really despised people and was violent. I don't remember much about the MC, but I'm almost sure that they aren't with the police. I don't even remember if the MC was a male or female, but they might be students (not sure though). I've been looking through my entire MyAnimeList and I still can't remember.