Nekomaru is a 300-year-old bakeneko, a supernatural monster cat originating from Shanghai. He journeys alongside Ryunosuke to Kyoto during Japan's transition from the Edo to Meiji period. His demeanor is timid, often cowering and whimpering at signs of trouble, a caution shaped by his frequent role as Gennai's experimental test subject.

Despite his apprehensions, Nekomaru wields potent supernatural abilities. His most notable power allows him to resurrect the deceased by sacrificing one of his nine lives, restoring them to perfect health, reinforcing his guardian role. He exhibits deep protectiveness and affection towards Ryunosuke, consistently prioritizing the young man's well-being and happiness, which motivates him throughout their travels.

Within Kyoto, Nekomaru operates under a government-enforced monster-registration system. His presence and abilities position him as a supporting figure in efforts to maintain peace alongside Ryunosuke, contributing to the management of unregistered monsters.