Hirune Mikura is the aunt of the protagonist, Mifuyu Mikura, and a key figure connected to the family library known as Mikura Hall. Unlike other members of her family who may have grown distant from the collection, Hirune is defined by her deep and singular passion for the books it houses. Her primary motivation and daily pursuit is to read every single volume within the family's vast and rare collection, a task she dedicates herself to with an almost all-consuming focus.

In terms of personality and daily life, Hirune is portrayed as a reclusive individual who essentially lives within Mikura Hall itself. Her absorption in the world of books means she is largely disconnected from practical, everyday affairs. She is described as being unable to handle routine tasks such as shopping for food or clothes by herself, leaving her dependent on her young niece for basic care and support. This dependency highlights a key aspect of her role in the story; while she is the elder, her vulnerability and fixation place her under the protection and responsibility of Mifuyu, adding to the protagonist's burdens.

Hirunes role in the narrative is central to the inciting incident of the curse. When a book is stolen from the library, she is consistently found in a deep, unresponsive sleep, clutching a strange talisman or piece of paper that reads "whoever steals this book shall be chased by the flags of magical realism". This state of suspended animation makes her a catalyst for the magical events that engulf the town. Her condition signals the activation of the curse and directly propels Mifuyu into action to recover the stolen books and save her aunt and the community. Her connection to the curse is so integral that speculation arises about her potential role in the thefts themselves, with theories suggesting she might be stealing the books not out of malice, but from a profound desire to physically enter and experience the worlds contained within them.

Regarding her development and notable abilities in the narrative, specific information is limited. Her primary function is as a representation of the librarys legacy and a direct link to the magical consequences of tampering with the books. Her character highlights the themes of obsession and the fine line between a deep love for literature and a withdrawal from reality. She lacks any overt magical or physical abilities, her power being her unwavering dedication to the act of reading itself. Her aunt-niece relationship with Mifuyu is the most significant connection, shaping the protagonists responsibilities and contrasting Hirunes own helplessness with Mifuyus reluctant but growing resourcefulness.