[Toullaty]

Spoiler for Volume 5:

From what I understood, Murohime recovers her childhood memory and unlocks her power. We discover that the sister of Haruomi was a big yandere.
During this time, his sister take over the body of Zilch and begins to be agressive. After a little fight, she runs away.
Later, Murohime kiss Haruomi and will join the other demons who was looking for Lucifer.
Zilch attacks their hide-out and fights with Murohime before to be stopped by Io. A discussion follows and everything resolves.
We find the trio later on the roof of the school, with the two girls fighting over their boy and looked by all the other girls and demons.