On the day of Yuto's 16th birthday, he and his childhood friend Rinko meet a mysterious katana-wielding girl on their way to school. The girl starts to lick Yuto face much to Rinko's shock, forcing her to drag him away. At school during a heated arguement on the rooftop, Yuto and Rinko are attacked by Taizo who is possessed by a demon spirit until the mysterious girl from earlier excorcises it and kills it. Later that evening, Yuto is met once again by the mysterious girl who introduces herself as the cat spirit Himari, and explains he is being targeted by ayakashi who are out to kill him and that she will protect him from them. However, Rinko appears unexpectedly and causes a ruckus after seeing them together. The following day, Himari transfers to Yuto and Rinko's school. The night before, Himari reveals that Yuto's family is one of the twelve Demon Slayer families that killed ayakashi for hundreds of years, and that one of her ancestors was spared by Yuto's ancestor that led to a vow for her kind to forever serve them. Refusing to accept this fact, Rinko challenges Himari to a sports battle for Yuto's custody, only for her to lose in all of them. After being told by Himari that she lacks the strength to protect Yuto, Rinko insults her by calling her a monster. After school, Yuto reconciles with Himari on the rooftoop, who asks if it is okay to have an ayakashi like her around him. Yuto, understanding that she is only concerned about his well-being 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 to her discomfort and it turns into yet another ruckus. Meanwhile, the ayakashi plan their move to kill Yuto when a mizuchi named Shizuki appears, claiming she will kill him herself.