Momoko Orizuka is vice-captain of the swimming team at Prefectural Umineko Commercial High School, known as Umishō. Though nominally second-in-command, she effectively leads the club due to the official captain Masa Ikariya’s unserious approach. This authority originated when she co-founded the swimming club with childhood friend Sanae Kise before Masa joined.

Her rational, serious, and mature nature makes her the perpetual straight man to the club’s eccentric members. She frequently erupts in frustration, particularly at Masa Ikariya’s antics—like his fixation on skimpy swimwear and body hair shaving—often resorting to physical discipline. Momoko enforces strict discipline and full effort during training, though the team’s chaotic environment routinely undermines her demands. Her zero-tolerance for nonsense and volatile temper intimidate the student body, earning her the moniker "Boss."

When transfer student Amuro Ninagawa arrives, Momoko challenges her directly to assess her swimming skills. Witnessing Amuro’s exceptional speed and raw talent convinces Momoko to aggressively recruit her as a key asset for competitive ambitions. Momoko herself excels in the 100-meter and 200-meter freestyle events, driven by a personal goal to qualify and succeed at the Kantou Tournament—a pursuit complicated by her team’s disarray and formidable rivals.

Her pragmatic leadership includes harsh logistical decisions, like relocating male members to a cramped storage room for changing to prioritize space for female teammates. While her background receives less focus than others, her family life appears conventional, contrasting with peers like Amuro. Post-graduation, she pursues a career as an elementary school teacher.