Nekomaru is a 300-year-old bakeneko (monster cat) serving as companion and guardian to Ryunosuke Sakamoto. He accompanies Ryunosuke from Shanghai to Kyoto to register as a legally licensed yōkai. His key ability allows him to transfer one of his nine lives to resurrect deceased individuals, restoring them to perfect health. He frequently uses this power to revive Ryunosuke after accidental deaths, though it occasionally misfires, such as temporarily placing Ryunosuke's soul into a female body.

Despite a timid and fearful personality, often cowering in confrontations, Nekomaru displays unwavering loyalty to Ryunosuke, prioritizing the young man's happiness and well-being. He possesses a large appetite, consuming substantial amounts of food, particularly at Sayoko's restaurant. His nervous disposition stems partly from being a test subject for Gennai Hiraga, the Shinsengumi's inventor, whose experimental gadgets often inflict painful or bizarre side effects on Nekomaru. He occasionally remarks that Gennai's ruthlessness makes the inventor seem more monstrous than himself.

A subplot reveals Nekomaru was originally given to Ryunosuke as part of an arranged marriage agreement. Ryunosuke was betrothed to Ranran, who died protecting him. Nekomaru resurrected Ryunosuke then, binding him to the condition that he must eventually marry Ranran. This arrangement surfaces later when Ranran reappears, creating romantic complications involving Ryunosuke and Kaoru Okita.

Nekomaru evolves from a comedic, cowardly figure into one capable of self-sacrifice. In the series finale, he expends his remaining three lives to resurrect Yuko Kondo, Toshie Hijikata, and Kaoru Okita after their deaths battling the antagonist Montague. This act reduces him to a non-corporeal spirit form. His final moments involve assisting Ranran and her panda companion Kankan in trapping Montague, enabling Okita to banish him permanently.