Momoe Sawaki is a 14-year-old girl undertaking Wonder Egg missions to resurrect her closest friend Haruka, who died by suicide. She is the niece of Shuichiro Sawaki, an art teacher at the school attended by other main characters. Tall, thin, and androgynous in appearance, she has short auburn hair swept left, dark green eyes, broad shoulders, and a well-toned figure. This leads to frequent misgendering, with others often mistaking her for a boy. Her typical outfit includes a navy blue sweater over a white dress shirt, loose beige slacks, navy blue socks, and burgundy shoes.

Her background centers on Haruka, the only person who recognized and affirmed her femininity during their friendship. However, Haruka later made romantic and sexual advances, requesting physical contact that terrified and confused Momoe, leading to direct rejection. Haruka died by suicide shortly afterward for undisclosed reasons, leaving Momoe with intense guilt and self-blame. Following this loss, Momoe adopted more masculine clothing and began using the masculine pronoun "boku," though she explicitly identifies as female and seeks recognition as a girl.

Momoe combines kindness, politeness, and selflessness with deep-seated gender insecurities. She displays bravery and justice, defending others against false accusations—such as protecting her uncle from claims he impregnated a student. Despite popularity among girls attracted to her masculine presentation, romantic attention based on being perceived as male distresses her; she views it as deceptive and reinforcing misgendering. Explicit references to her as a man by Wonder Killers or others cause visible anguish, sometimes leading her to cry at her reflection. Her sexuality remains ambiguous, with implied attraction to boys but no explicit rejection of interest from girls.

In Wonder Egg missions protecting girls who died by suicide, she encounters patterns worsening her gender struggles. Early missions with Miwa and Mizuki involve both misinterpreting Momoe as male, developing romantic feelings, and confessing love before departing—Miwa nicknames her "Momotaro," a masculine folk hero. Later, she bonds with trans boy Kaoru Kurita, who knows she is a girl yet expresses romantic interest and kisses her. Unlike previous cases, Momoe does not refuse Kaoru, suggesting potential complexity in her feelings or identity. She wields an umbrella with a spearhead in these dream worlds and bonds with Panic, a crocodile-like creature emerging from her jeweled necklace Item.

Her development includes forming friendships, particularly with Ai Ohto. After being misgendered by Neiru and Rika, a distraught Momoe asks Ai to identify her gender; Ai immediately affirms she is a girl, bringing visible relief and prompting Momoe's temporary use of "watashi." They bond over shared experiences like discussing Adam's apples. Momoe reveals aspirations to become a dietician and joins group activities like painting nails. She defends her uncle against ongoing suspicions tied to another character's suicide, sometimes causing group conflict.

Her experiences culminate in a failed first date where the boy, who asked her out, panics upon realizing she is a girl. In quieter moments, like conversations with Panic, Momoe expresses openness to loving others regardless of gender, indicating ongoing exploration of identity and relationships.