Shiori Kitahara is the younger sister of the protagonist, Iori Kitahara. She is a third-year middle school student, making her several years younger than her brother who is away at university. In terms of appearance, Shiori shares her brother’s blue hair and is almost always seen wearing traditional Japanese clothing, specifically kimonos. This style is not necessarily a personal preference but is done to uphold the family's image as owners of a traditional inn, as her parents expect her to present herself in this manner.

On the surface, Shiori presents herself as the ideal younger sister. She is gentle, quiet, polite, and speaks in an elegant, formal manner that makes her seem far more composed than a typical student her age. Her initial interactions with Iori are filled with affectionate concern for his well-being, and she often calls him "brother" in a sweet and caring way. However, this demure and loving persona is largely a calculated act. Beneath her polite demeanor lies a cunning and manipulative side. She has no desire to inherit and run the family inn, a burden her parents are placing on her. Lacking the courage to openly defy them, she has devised a secret plan to have her brother, Iori, take over the family business instead.

Shiori’s primary motivation is to escape the obligation of managing the family ryokan. To achieve this, she travels to Iori’s university town under the pretense of being a worried sister checking up on him. In reality, her mission is to convince or coerce him into returning home and accepting ownership of the inn. She employs various tactics to achieve this goal, including cooking meals designed to invoke homesickness and feigning a brother complex to manipulate his sense of family duty. She is also highly skilled with technology, having installed a hidden camera disguised as a stuffed rabbit to monitor Iori’s activities from a distance, confirming that his letters home were lies about his respectable university life.

Her role in the story is that of a catalyst who brings the pressures of the Kitahara family business directly into Iori’s new life. Her arrival at the Grand Blue diving shop creates a temporary disruption, as the other characters, particularly the otaku Kouhei Imamura, become enamored with her polite exterior. Despite her scheming nature, a genuine development occurs when she is convinced to try diving. During this experience, away from the expectations of her family, she is able to let her guard down and simply enjoy herself. This moment of sincere fun is a stark contrast to her usual calculated behavior and hints at her own repressed desires.

Key relationships define Shiori’s character. Her most significant relationship is with her brother, Iori. While she attempts to manipulate him, he is often too dense to fall for her act. In a private moment, Iori reveals that he knows she does not want to run the inn either, a moment of honesty that deepens their bond and shows a mutual understanding beneath their scheming. Her relationship with her mother is also notable, as it is the source of some of her more unusual abilities. Her mother has taught her special self-defense techniques that involve the use of chloroform, a skill she does not hesitate to use on her own brother. Regarding other characters like the diving instructor Nanaka Kotegawa, Shiori is perceived simply as a polite and sweet girl, with the others remaining unaware of her more calculating true nature.

Beyond her deceptive skills and tech-savviness, Shiori’s most notable ability is her mastery of dual personas, seamlessly switching from a sweet, traditional little sister to a determined strategist. Her chloroform technique, inherited from her mother, stands out as a particularly unique and practical skill. Ultimately, despite failing in her primary goal of bringing Iori home, her visit concludes with her having enjoyed the experience of diving, suggesting a personal growth that extends beyond her family-defined role.