Junpei Kōsaka is the main protagonist of his story, a second-year high school student whose life is complicated by a significant paradox: he suffers from a severe allergy to cats and, as a result, harbors a deep resentment toward them, yet almost everyone in his life adores felines. This includes his mother and younger sister, who dote on the family’s ill-tempered cat, Nyamsus, and, most critically, his classmate Kaede Mizuno, the girl on whom he has a hopeless crush. His birthday is on January 1st.

Junpei’s already frustrating situation takes a supernatural turn when, on his way home from school one day, he kicks an empty can that ricochets and decapitates the statue of a local cat guardian deity. As punishment for this act, he finds himself cursed. The curse first manifests as the ability to understand the speech of cats, a power he reluctantly confirms by conversing with his own family’s cat, Nyamsus. To break the curse and avoid its final, fatal stage, Junpei must successfully fulfill one hundred wishes from cats. The consequences of failure are dire: he will be transformed into a cat himself and subsequently die from his own severe cat allergy. Adding to the pressure, the curse comes with a critical caveat. If another human being discovers his supernatural predicament, the curse will become stronger and accelerate faster. The person who discovers the secret will also become more unlucky and accident-prone.

In his daily life, Junpei presents as a fairly normal and kind-hearted young man, though misfortune seems to consistently find him. He genuinely cares for the people around him but can become easily frustrated and flustered, particularly when cats are involved. Despite his outward frustrations and his own dangerous allergy, his inherent kindness often wins out. He takes on the requests of cats, even when they put him in perilous or absurd situations, and has demonstrated a willingness to help others, such as the eccentric Nagi Ichinose with her difficult requests. This helpful nature is central to his role in the story, as he is forced to juggle his normal life as a high school student with his secret, mandatory quest as a servant to the local feline population. He is socially awkward, especially around his crush, Kaede Mizuno, and can be remarkably dense when it comes to recognizing the romantic interest that other girls show him.

Junpei’s key relationships form a significant part of his character's arc. His primary romantic interest is Kaede Mizuno, an athletic and cat-loving girl he met during their first year of high school in an awkward encounter when she fell on him while trying to rescue a kitten. He is too shy to confess his feelings to her, though she begins to develop feelings for him in return as the story progresses. He also has a complicated history with Kanako Sumiyoshi, a childhood friend. After a misunderstanding involving a cat doll in elementary school, their relationship became hostile, and they stopped speaking. When they reunite in high school, Kanako is brash and rude toward him, though she secretly harbors romantic feelings for Junpei, which he is slow to recognize. He later encounters the Kirishima twins, Kotone and Akari, the daughters of the Buddhist monk at the temple where the cat statue resides. Both are aware of his curse and assist him in various ways. Kotone is sweet on the surface but has an obsessive, almost sadistic interest in Junpei and his misfortunes, while Akari is more asocial and tsundere, initially suspicious of him but gradually developing her own feelings. Other significant figures include Nagi Ichinose, a masculine, energetic upperclassman who falls for Junpei after he comforts her, and Chizuru Mochizuki, a college student mail carrier with no sense of direction who enjoys teasing him and eventually becomes a part-time coworker.

Over the course of the narrative, Junpei exhibits some development, though he largely remains a normal guy trying to navigate his extraordinary circumstances. He slowly becomes more aware of the romantic feelings other girls have for him, for instance beginning to consider Kanako more than just a rude childhood friend. While his main affection remains with Kaede Mizuno, he develops bonds with each of the girls in his life in different ways. He is indecisive at times, often worrying excessively about the positions and feelings of others, but he shows a consistent streak of bravery and selflessness when it comes to helping others, both human and feline, despite the personal risk. His defining ability is his curse-given power to understand and communicate with cats, which is the primary tool he uses to complete his required one hundred good deeds and the driving force behind most of his adventures.