Natsumi Onitsuka is a student at Yuigaoka Private Girls High School, starting as a first-year in the second season before becoming a second-year. Born on August 7th (Leo), her associated color is the distinctive "oninattsu" pink. She joins the school idol group Liella! and its sub-unit 5yncri5e!. Her surname, Onitsuka, combines "demon/ogre" (鬼, oni) and "mound" (塚, tsuka), while her given name, Natsumi, means "summer" (夏, natsu) and "beautiful" (美, mi).

Before Liella!, she pursued online influencing and streaming with a self-centered, opportunistic focus, prioritizing monetization. This included undisclosed appropriation of revenue from promoting Liella!'s videos. Her behavior stemmed from repeated failures in past endeavors—like winning a running medal, achieving a Nobel Prize in science, or modeling—which ended in disappointment due to perceived inadequacies in speed, academics, or stature. These failures fostered cynicism, leading her to prioritize financial security over dreams or connections. Her personality initially reflects egotism and greed, fitting Liella!'s "Mean" archetype and a choleric temperament marked by aggression and dominance; she also embodies "The Cynic" within the group.

A turning point occurs when Liella! confronts her exploitation. Instead of ostracizing her, they invite her to share their collective dream, with Kanon Shibuya recognizing Natsumi's underlying potential. During this confrontation, Natsumi displays uncharacteristic concern, admonishing younger students considering abandoning their aspirations based on her own understanding of perseverance.

After joining Liella!, she continues streaming but redirects it to support the group. Her acceptance is symbolized by participating in Liella!'s pre-performance cheer, culminating in a nine-point star formation signifying the completed lineup. Her development concludes with a full Heel–Face Turn, transitioning from self-serving to a committed member valuing teamwork and shared goals.