Cat and Girl and Allergy.On the day of Yuto Amakawa's 16th birthday, he and his childhood friend Rinko meet a mysterious katana-wielding girl on their way to school. The girl starts to come on to Yuto to Rinko's shock, causing her to drag him away. At school during a heated argument on the rooftop, Yuto and Rinko are attacked by their friend Taizo who is possessed by a demon spirit until the mysterious girl from earlier exorcises it and kills it. Later that night, Yuto is visited by the mysterious girl who introduces herself as the cat spirit Himari, and explains that he is being targeted by various ayakashi who are out to kill him and that she will protect him from them due to an ancient vow. However, the appearance of her cat ears and tail triggers Yuto's cat allergies, and at the same time Rinko appears unexpectedly and causes a ruckus after seeing them together. The following day, Himari transfers to Yuto and Rinko's school. In a flashback from the night before, Himari reveals that Yuto's family (i.e., the Amakawa Family) is one of the twelve Demon Slayer families that has protected mankind from ayakashi for hundreds of years, and that one of her ancestors was spared by Yuto's ancestor, leading to a vow from her kind to forever serve them. Refusing to accept this fact, Rinko challenges Himari to a sports battle (which includes running, high jump, kendo, judo and tennis), only for her to lose in all of them despite being the school's best athlete. After being told by Himari that she lacks the strength to protect Yuto, Rinko insults her by calling her a monster. On the school rooftop, Himari asks Yuto if it is okay to have an ayakashi like her around him. Yuto, understanding that she is only concerned about his well-being just like his grandparents were, allows her to perform her duty. The next morning, Rinko comes to apologize to Yuto, only to see Himari in his bed again, much to her annoyance and it turns into yet another ruckus. Meanwhile, a mizuchi or water deity named Shizuku appears to the other ayakashi, claiming she will kill Yuto herself.