[Xacual]

Originally Posted by 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.

Spoiler for The Killer:

She kills people and takes their souls and uses them like extra lives for herself. That's another reason she was confused about Ayumu because she didn't get her extra life as it were. It has been causing problems for the Underworld because people are dying but no souls are at the murder sites to be collected.

Link is a translation to chapter 3 in volume 1 where it's somewhat mixed around from the anime. It seems like Ayumu confronted Yuu about the murder thing at the end of chapter 2 and Sera took the two to the bowling alley to try to cheer up Yuu. Which is where Ayumu and Orito ran into them. Massively cute Yuu moment in this too. Also includes Ayumu talking to DaiSensei about Haruna and Yuu explaining her powers to Ayumu. Also explanation of what Megalos are.
http://pastebin.com/Gu3K8Bv4