Aruto is the second of the three sisters who manage Kokoro Library, a small facility nestled in a remote mountain region. She is the middle child, with an older sister, Iina, and a younger sister, Kokoro. Among the three siblings, Aruto is distinguished as the tallest and is often characterized as having a wild or pragmatic personality, in contrast to the more reserved Iina and the earnest but clumsy Kokoro. She is depicted as a bookworm and an intellectual, serving as a grounded and realistic presence within the family. Aruto is also noted to be an excellent cook.

Beneath her practical exterior, Aruto harbors a significant secret life. She is the author of a popular romance novel series, which she writes under the pen name Himemiya Kirin. Her writing is prolific, with some accounts suggesting she produces a book every month. To maintain her anonymity and separate this persona from her daily life, Aruto adopts a disguise when she writes, which includes wearing a blonde wig. This hidden profession is a central aspect of her character, as she goes to considerable lengths to keep it a secret from her sisters, particularly because Kokoro is an avid fan of the Kirin Himemiya novels. The need to stay up late into the night to write is the reason she is known to be a heavy sleeper during the day. Aruto is also identifiable by her glasses.

Aruto's role in the story is that of a protective and capable elder sister who works alongside her siblings to keep the library running after the death of their parents. She is deeply involved in the day-to-day operations and shares her sisters' concern about the library's low number of users and its potential closure. Her key relationships are primarily with her sisters, Iina and Kokoro. While she is the saner and more stable foil to Iina, who has an intense and often uncomfortable obsession with Kokoro, Aruto shows a more typical, if still somewhat distant, sibling concern. One of her central character conflicts arises from her double life; she must balance her responsibilities as a librarian and sister with her passion for writing, all while trying to prevent her family from discovering her secret identity.

Throughout the narrative, Aruto's personal development is tied to her secret coming to light. In one instance, Kokoro wins a contest to meet her favorite author, Kirin Himemiya, and is brought face-to-face with Aruto in her disguise without recognizing her. This situation creates dramatic irony and forces Aruto to confront the consequences of her secrecy. Ultimately, her hidden talent contributes to the library's legacy, as she writes a book titled Kokoro Library, a story about a library where miracles happen, which is delivered in the final episode. Among her notable abilities, besides her culinary skills and her talent for writing popular fiction, Aruto is also noted to have stated that she does not believe in a god, marking her as a character with a pragmatic and secular worldview within the story's setting.