Nanami Kento isn't a wizard, but he knows how to fight curses. A cursed spirit called Mahito has appeared, killing and transforming people. Mahito can also transform himself. Nanami hates him, a lot. Mahito keeps on attacking Itadori and Nanami, which irritates him. Nanami keeps fighting and trying to kill Mahito but no, it doesn't work. Mahito finds a way to survive every time, so Nanami decides to do something to distract Mahito from attacking them. Kidnapping his sister. Nanami knew his sister was important for Mahito. His sister was the same as Mahito, just different genders and height. Nanami didn't knew Gojo would be pleased with this but oh well.