Tom Kusanagi is an eleven-year-old boy, the sole biological son of the Kusanagi family. His discovery of an orange stray cat leads him to bring it home, where his family adopts it. Inspired by its fur color, Tom names the cat "Mikan," referencing the Japanese word for mandarin orange. He becomes the first family member to uncover Mikan's unusual abilities: walking upright on hind legs and speaking human language. Observing suspicious behavior like Mikan refusing breakfast while household sake diminishes, Tom investigates and confirms the cat's extraordinary traits.

Learning Mikan's backstory—including life with a previous owner named Tatsuzō who died of illness, followed by human rejection due to fear—Tom insists Mikan reveal his abilities to Tom's parents. He passionately argues for Mikan's acceptance despite his nature, successfully persuading his initially hesitant parents to fully welcome Mikan while agreeing to keep his secret. Tom attends Yozakura Elementary School and assigns Mikan the task of keeping a picture diary ("enikki") documenting daily life, a recurring element in their interactions. Though he frequently quarrels with Mikan, Tom maintains a deeply caring and protective relationship with his adoptive brother.

Tom's family includes his father, Tōjirō, a computer engineer, and his mother, Kikuko, a homemaker. While the manga portrays his parents as consistently understanding, anime adaptations introduce moments of familial tension, such as parental arguments. Tom later gains a younger sister, though her role remains minimal in documented narratives. His social circle expands to include friendships, like one with a character named Sera. As the story progresses, Tom participates in significant events involving Mikan's life, including Mikan's marriage to another cat, Killy, and the family's adoption of one of their offspring, Ringo, who also develops human speech abilities.