Onee-san, known as "The Lady," works as a dental assistant in a quiet town, forging a profound bond with a reserved young boy, Aoyama. Her past is shrouded in ambiguity, with fragmented memories of a coastal childhood she later doubts. She wields unexplained abilities to transmute objects—soda cans into daylight penguins or nocturnal bats—powers tied to a time-warping spatial anomaly called "the Ocean," a sphere of energy she struggles to comprehend.

Cheerful and warm, her playful nature contrasts with Aoyama’s introversion. They share a platonic, mentor-like relationship, collaborating to unravel the mysteries of the penguins and the Ocean while honing his chess skills. Though outwardly human, her lack of need for sustenance and physical decline when separated from the Ocean hint at an otherworldly essence.

Her ties to the Ocean intensify as the narrative unfolds, revealing her origins within its enigmatic realm. Survival instincts inadvertently spawn destructive Jabberwock-like entities from her childhood nightmares, endangering the town. In a climactic act of self-sacrifice, she devises a plan with Aoyama to obliterate the Ocean, saving the community at the cost of her existence. Her fate lingers unresolved, though subtle clues—like a penguin returning Aoyama’s lost toy—hint at a lingering presence in another dimension.

Mirroring Princess Kaguya’s celestial yearning, her arc grapples with belonging and longing. She shields others from bullies, comforts Aoyama in illness, and prioritizes collective safety over her survival. Her bond with Aoyama, rooted in mutual respect and transcendent understanding, catalyzes his emotional growth, leaving an indelible mark even as she vanishes, a testament to selflessness and the ephemeral nature of connection.