Hotaru Yūyami is a horror picture book author with ten years in the profession. His motivation stems from a childhood desire to convey feelings of "love" he couldn't express to his parents. He experienced significant early loss: both parents died when he was young, leading his grandfather to raise him. This grandfather also passed away during Hotaru's university years. During this time, his grandfather's dog, Hanako, provided emotional comfort and healing. Hanako's later death from illness caused Hotaru to lose motivation for both work and daily life.

The story opens on the culturally significant 49th day after Hanako's death, marking the end of mourning when the spirit is believed to move on. Struggling to let go, Hotaru clings to Hanako's red collar and belongings as symbols of grief. Amid this emotional state, he discovers a pink mushroom growing in his garden. This mushroom transforms into a creature called Kinokoinu Mushroom Pup, becoming a catalyst for Hotaru's healing journey. Kinokoinu understands human conversations, shows curiosity about its surroundings, and attempts subtle gestures to comfort Hotaru.

Physically, Hotaru is an adult male with black hair cut to ear length and black eyes. He typically wears sweaters, jeans, and sneakers. His relationships include childhood friends Komako Amano, who becomes his editor, and Tsubaki Uehara. Other connections are Itsuki Yara, Plum, Anzu Uehara, Tsubaki Uehara, and Ayumi Amano.

Hotaru's character development revolves around processing grief and rediscovering purpose. Kinokoinu's appearance initiates a gentle progression toward emotional recovery. The creature's presence encourages Hotaru to confront not only Hanako's loss but also unresolved pain from his parents' deaths. His journey is a quiet, introspective exploration of mourning, depression, and gradual forward movement, with Kinokoinu serving as an unexpected companion helping him re-engage with life.