Onee-San, known as The Lady, works as a dental assistant in a small town. She befriends a reserved boy named Aoyama, teaching him chess during spare moments at a local café. Her upbeat, friendly, and nurturing personality contrasts with his quiet nature. She develops genuine fondness for Aoyama, supporting his scientific curiosity with consistent kindness and concern.

She mysteriously generates living penguins from ordinary objects like soda cans, an ability she initially doesn’t understand. Partnering with Aoyama, she experiments to uncover its origins, deepening their bond. She permits Aoyama overnight stays at her apartment, though they later lose contact until reuniting at a summer festival months afterward.

Her protective instincts surface upon finding Aoyama bullied in a forest; she summons penguins to frighten the attackers away. As supernatural events escalate—including an expanding anomaly called "The Ocean" and destructive Jabberwocky entities—she accompanies Aoyama on a seaside train journey. She falls ill during the trip but later tends to Aoyama when he succumbs to sickness, comforting him.

When a research team becomes trapped within The Ocean, she takes decisive action despite her failing health. Joining forces with Aoyama, she rides a wave of penguins into the anomaly to rescue them. At the story’s climax, she destroys The Ocean to shield the town, vanishing after a final embrace with Aoyama at the café. Her ultimate fate remains intentionally unresolved.

Throughout, she exhibits unwavering compassion—shielding another character, Uchida, with her penguins and soothing Aoyama during illness. Her relationship with Aoyama forms the emotional core of her journey, defined by mentorship, shared investigation into her powers, and a poignant, self-sacrificial resolution.