Nanami Kiryuu is a student at Ohtori Academy and the younger sister of Touga Kiryuu. She is in the seventh grade, approximately twelve to thirteen years old. Her background includes being adopted into the Kiryuu family alongside her brother, though she is unaware of this fact for most of the story. This hidden truth underpins her intense, obsessive love for Touga, which borders on romantic and possessive. She views any girl who attracts his attention as a threat, especially Utena Tenjou and Anthy Himemiya, and frequently devises schemes to humiliate or punish them. These plans often backfire in bizarre, animal-related ways, such as a gradual transformation into a cow after receiving a cowbell from a secret admirer.

Her personality is a mixture of arrogance, insecurity, and volatility. She presents herself as a queen bee, snobbish and self-centered, and commands a trio of sycophantic followers. Yet her need for validation from her brother and her peers reveals a deep fragility. She can be cruel, but she also shows moments of empathy and vulnerability, particularly in episodes focused on her own struggles. Her motivations evolve over the series. Initially driven by jealousy and a desire to maintain her exclusive bond with Touga, she later steps into the role of Student Council President after Touga is wounded. This position allows her to explore her own identity separate from him. As she witnesses the abusive dynamics between Akio and Anthy, she becomes disillusioned with the dueling system and the false promise of eternity. She eventually discards her student council ring and uniform, refusing to participate further.

Her role in the narrative shifts from an early antagonist and comic relief to a character who recognizes the flaws in the world around her. She becomes a voice of disillusionment, attempting to warn Utena against the final duel. Key relationships include her obsessive bond with Touga, her dismissive dynamic with the younger Mitsuru Tsuwabuki, and her evolution from bullying Anthy to understanding her suffering. Her development is notable for its speed: she begins as one of the most immature characters and ends as one of the most clear-eyed, choosing to walk away from the system.

In combat, Nanami is a skilled duelist who wields a scimitar and a dagger in a dual-blade fencing style. She relies on quick strikes and strategic footwork, though her emotional vulnerabilities can undermine her confidence. Her most distinctive trait is her constant, often absurd, entanglement with animals and bizarre occurrences, which serve as both humor and a reflection of her inner turmoil.