Kinokoinu originates from a pink mushroom that spontaneously sprouted in Hotaru Yuyami's garden soon after his dog Hanako's death. This fungal organism uproots itself, transforming into a small dog-like form adorned with distinct pink-and-white spotted markings across one ear and facial side. Its unblinking black dot eyes and ability to walk upright on hind legs contrast its canine appearance.

Exhibiting sapience beyond animal intelligence, Kinokoinu comprehends human speech and emotional cues instantly. It communicates nonverbally through body language and vocalizations, later manipulating objects like crayons to draw and write. The entity demonstrates persistent determination and mischievous tendencies, intervening in human affairs through methods ranging from therapeutic to disruptive—such as destroying Hanako's sentimental objects or defacing creative works to halt prolonged grief or stagnation.

Biologically fungal, Kinokoinu releases substantial atmospheric spores, leading to the emergence of a similar entity named Plum, establishing kinship. Despite its mycological nature, it prepares and consumes human food, showing particular fondness for takoyaki.

Kinokoinu's bond with Hotaru evolves from an unconventional pet replacing Hanako to a guardian-like familial role, actively facilitating Hotaru's emotional recovery and social reintegration through interventions and inherent presence. It forges connections beyond Hotaru—including Komako Amano (Hotaru's editor) and mycologist Yara—while nurturing relationships with other mushroom-based beings like Plum. Its presence gradually expands Hotaru's social world by attracting visitors and fostering community interactions.