Ai Ōto is a 14-year-old girl distinguished by heterochromia – one golden-yellow eye and one ocean-blue eye – a trait that subjected her to severe bullying. Her dark blue hair features a prominent ahoge and a yellow triangular hair clip above her left eye, which she often covers with her bangs due to self-consciousness. She typically wears a bright yellow hoodie adorned with a sunflower design, volleyball shorts, and blue-and-white platform sneakers. This hoodie's egg-like appearance when worn up symbolically reflects her role in protecting others within the dream world. Her personal space, particularly her room, serves as a sanctuary decorated with colorful flags, maps, plants, and symbolic flora like calla lilies (representing rebirth) and morning glories (symbolizing unrequited love or fleeting bonds), emphasizing her longing for safety and connection.

Once a lonely outcast, Ai's life changed when transfer student Koito Nagase befriended her and admired her heterochromia. This friendship became Ai's sole emotional anchor, though she harbored a precocious crush on her homeroom teacher, Shūichirō Sawaki. After witnessing Koito endure bullying, Ai attempted to document the abuse but froze in fear, failing to gather evidence. This inaction compounded her guilt when Koito died by suicide, an event initially presented as a jump from the school roof. Consumed by self-blame, Ai withdrew from school and isolated herself, confining most of her time to her room. Her capable, caring mother remained emotionally distant, never entering Ai's private space, reinforcing her solitude.

A mysterious encounter led Ai to a gachapon machine dispensing "Wonder Eggs." Upon cracking one open, she entered a dream world where she was tasked with protecting the girl inside from monstrous manifestations of trauma known as "Seeno Evils," "Haters," and "Wonder Killers." Guided by entities called Acca and Ura-Acca, Ai learned that saving enough souls in this realm could resurrect Koito. Her primary weapon is a four-color ballpoint pen that transforms into a large club, though she adapts objects from the rescued girls into weapons, such as a gymnastics ribbon used as a whip or penlights wielded as blades. She later receives a pomander that hatches into a chameleon-like companion named Leon. This protector role ignites fierce determination in her, marked by her catchphrase, "Now I'm really mad!" during battles.

Her quest intersects with three other girls bearing similar burdens: Neiru Aonuma, a stoic child CEO seeking to revive her sister; Rika Kawai, a former idol burdened by guilt over a fan's death; and Momoe Sawaki, Sawaki's niece struggling with gender identity and loss. Initially withdrawn, Ai gradually opens up to them, forming a bond that evolves into genuine friendship. Their interactions in Ai's room or other gathering spots become crucial emotional support, though discussions often confront painful topics like Sawaki's ambiguous role in Koito's death. Ai's growth shifts her from passive guilt to active empathy, notably comforting Minami Suzuhara, an egg girl confronting her abuser, by declaring, "You want someone to find you and love you! So I won't leave you alone!" This resolve extends to her personal life; she confronts her unresolved feelings for Sawaki after he becomes engaged to her mother, consciously choosing to support their relationship.

In her final mission to resurrect Koito, Ai encounters a parallel version of herself from another world. This alternate Ai never met Koito, leading to intensified isolation and eventual suicide by drowning in the school pool. During a shared battle against Sawaki's monstrous form and Frill's creations, the parallel Ai sacrifices herself, taking a boomerang strike meant for the original Ai and losing her blue eye. Though Ai succeeds in reviving Koito, the reunion is tragic: Koito shows no recognition, dismissively referring to Ai as "the model for Mr. Sawaki's painting." Heartbroken but resilient, Ai transfers schools and later discovers Neiru has vanished after making a deal with Frill. In response, Ai reignites her mission, purchasing a new Wonder Egg to rescue Neiru, demonstrating her enduring commitment to saving others despite loss.

Her development centers on transcending survivor's guilt and passivity. Early traits of naiveté and emotional withdrawal—such as misunderstanding adult topics like "junior idols"—give way to courage and self-awareness. She rejects her former tendency toward "bystander syndrome," declaring she is "done pretending not to see." This evolution is mirrored in her symbolic associations: the sunflower on her hoodie signifies resilience, while her room's flowers reflect themes of purity, rebirth, and unresolved longing. The parallel Ai's sacrifice underscores how Koito's original act of friendship provided the foundation for her resilience, contrasting the tragic fate of an Ai devoid of that bond. Her story concludes not with closure, but with a reaffirmed dedication to fighting for those she loves, embodying persistent hope amid ongoing trauma.