[SoFarGone]

Spoiler for the serial killer:

Kyoko is the killer. I believe she murdered people because she needed power to stay alive or something correct me if I'm wrong. She is a dark mahou shoujo or one of the necromancer allies of Yuu. The reason she wanted to see Ayumu isn't because she had a crush on him its because she wants to confirm if he is really alive since she thought she killed him.