Dandadan. Volume 16: 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!
Momo and Okarun task Turbo Granny with uncovering who picked up Okarun’s family jewel from the police station. She succeeds in infiltrating the station but is quickly thwarted by its computer system. Undaunted, the pair decide they’ll try again, but on their way home, they run into the Serpoian Rokuro, and it’s through his powers that they’re able to identify the culprit! But why is it that guy?