Dandadan. Volume 10: Yukinobu Tatsu

A nerd must fight powerful spirits and aliens all vying for the secret power of his “family jewel,” so who better to fight alongside him than his high school crush and a spirit granny?!
Momo Ayase and Okarun are on opposite sides of the paranormal spectrum regarding what they’ll believe in and what they won’t. Their quest to prove each other wrong leads them down a path of secret crushes and paranormal battles they’ll have to participate in to believe!
To avoid being late for school on Vamola’s first day, Momo takes the mysterious kaiju girl on a shortcut through some off-limits ruins. But the protection spell they perform doesn’t work, and she runs into the slit-mouthed woman who resides there! The two narrowly escape, and it’s only later that Momo realizes the real danger they were in. So why is it that for Momo’s own safety, Seiko is now forbidding her from leaving her room after 10 p.m. for the next week?!