Shizune Seo attends Reien Girl's Academy as a student and becomes Azaka Kokutou's roommate. Her family operates a wine cellar and owns a collie dog named Akira. She shares a hobby with Akira Seo and pursues it passionately.

Shizune possesses Mystic Eyes of the "Prediction" type, granting her precognition to see future events. Her ability typically reveals occurrences up to three days ahead, though she occasionally experiences flash-forwards from more distant timelines. This power causes her significant emotional distress due to her belief that the futures she witnesses are absolute and unchangeable. She feels guilt over leveraging her ability for academic advantage, such as previewing exam answers to secure top grades, and over involuntarily glimpsing others' private futures without consent. Her fatalism originates from experiences like twice foreseeing her dog's death yet being unable to prevent it, leading her to view the power as a curse.

Her character development initiates upon meeting Mikiya Kokutou. After she foresees a construction worker's death, Mikiya intervenes and saves the man, disproving the inevitability of her visions. This challenges her beliefs. During a conversation at Cafe Ahnenerbe, Mikiya reassures her that her ability extends ordinary human prediction and advises reframing her perspective. The interaction alleviates her despair and sparks a slight romantic attraction. However, upon encountering Shiki Ryougi, Shizune recognizes—through both her powers and intuition—that she cannot rival Shiki's presence in Mikiya's life. She subsequently warns Mikiya that Shiki may lead to his death, but he rejects the warning, accepting his fate for Shiki's sake.

Later, when Shizune meets Azaka and shakes her hand, she foresees they will become inseparable friends. After Azaka resolves a case during a school break, Shizune expresses gratitude, having predicted the outcome without knowing specifics. Shizune's prediction about Mikiya's potential demise resurfaces when he recounts it to Shiki after surviving Lio Shirazumi's attack.

Her ability's mechanics clarify through contrast with other precognition types. Unlike "Calculation"-type precognition, which actively shapes futures, Shizune's "Prediction" type passively observes them. The emotional burden of her power—experiencing events like deaths twice and feeling detached from life's surprises—gradually lessens after her encounter with Mikiya, leading her toward a more positive outlook.

Shizune was incorporated into early drafts as the protagonist of "Future Gospel," a narrative centered on her conversation with Mikiya. Though minimally active in the main storyline, her conceptual framework influenced a similar character in a Tsukihime side story.