Dandadan. Volume 4: 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!
The banana-stealing Serpoians are back! And they’ve trapped Okarun, Momo, and Aira in a void to have another go at stealing their reproductive organs. The subsequent battle causes the powers of the Acrobatic Silky dwelling within Aira to awaken, and she temporarily joins forces with Momo and Okarun. But will they be any match for their opponents after the aliens merge with their minions to form an even greater being?!