Miku Miyama is a young girl living in the small countryside town of Tana. She is the younger sister of Miki Miyama, who works at a local store. Miku is frequently seen carrying a frog doll with her, which she sometimes changes into a tadpole form, a whimsical habit that reflects her youthful and curious nature. She possesses an extensive knowledge of the mysterious fairies—also referred to as material fairies—and the enigmatic boy named Wakkun, who has not aged since childhood. This knowledge makes her a key figure in the unfolding events of the story.

Her personality is marked by a quiet perceptiveness and a willingness to engage with the supernatural elements that surround her. She is not overtly energetic but displays a steady, observant demeanor. Miku’s motivations are rooted in helping the protagonist, Ayumu Aizawa, recover his lost childhood memories. When Ayumu visits Tana and meets Miku by chance, she becomes the catalyst that sends him on a search for Wakkun, triggering the central mystery of the series. Her role in the story is that of a guide and a bridge between the ordinary world and the strange phenomena involving Wakkun, Dosshiru, and Shisshin.

Miku’s key relationships include her older sister Miki, who provides a grounding family presence, and Ayumu, whom she assists through her understanding of the fairies. She also has a direct connection to Wakkun and the fairy-like entities, which sets her apart from other characters. Over the course of the narrative, Miku’s development is tied to the gradual unraveling of the secrets of Tana; she becomes more involved in the investigation and demonstrates a growing confidence in her abilities as a source of insight. Her notable abilities are not physical or magical in a flashy sense, but rather her deep awareness of the local legends, fairies, and the history that links Ayumu to his past. This intuitive and learned expertise proves essential in guiding Ayumu and others toward understanding the truth of the summer he cannot remember.