Mikan, originally named Tom, was abandoned as a kitten alongside his siblings before being found and adopted by an elderly man, Tatsuzō. Living with Tatsuzō, he gained the ability to speak human language and walk upright on his hind legs. After Tatsuzō died from illness, Mikan was forced to leave his home when neighbors perceived him as a supernatural entity. He wandered while hiding his abilities until encountering a boy named Tom Kusanagi, who brought him home. The Kusanagi family adopted him, renaming him Mikan—derived from the Japanese word for "orange fruit"—due to his orange fur. Discovering his unique traits, the family reacted with shock but gradually accepted him under the condition his abilities remain secret.

Physically, Mikan is an orange cat with a white muzzle, paws, and tail tip, and green eyes. He occasionally wears accessories, like a leaf-patterned necklace. His personality blends cheerfulness and mischievousness, often teasing his adoptive brother Tom. He displays artistic tendencies, particularly through maintaining a picture diary assigned by Tom, documenting his daily life with simple hiragana and illustrations. Emotionally sensitive, he experiences insecurity about familial acceptance and lingering grief over Tatsuzō’s death. He frequently drinks sake, a habit started with Tatsuzō, sometimes becoming intoxicated. In the manga, he is generally well-behaved despite curiosity, while the anime portrays him as more impulsive and prone to causing trouble, like disrupting others by playing the harmonica.

His relationships include a deep bond with the cherished memory of Tatsuzō. Within the Kusanagi family, he shares a sibling-like dynamic with Tom, characterized by frequent squabbles but mutual care. He befriends neighborhood cats Botan and Kuroguchi, and later forms a romantic relationship with Killy (or Kelly). They have kittens, one of whom—Ringo—inherits Mikan’s ability to speak and walk upright. Ringo is adopted by the Kusanagis, briefly causing Mikan to lose his own speech before recovering it.

Mikan’s development involves navigating human society while concealing his traits, forming a family, and confronting challenges like temporary separation from Killy during her pregnancy in the anime. The manga further explores fantastical elements, including journeys to magical realms with dragons, not adapted into the anime. He evolves from a solitary, traumatized cat to one integrated into a human family and feline social circles, ultimately becoming a father and mentor to Ringo.