Izumi Nijō is a first-year student at Senriyama Girls High School, a prestigious institution known for its strong mahjong program. She competes for the school as the sergeant, the second player in the team rotation, and her primary playing style is defensive and stable. With a birthday on November 8, Izumi is recognizable by her short, dark brown hair and brown eyes. Her confidence is a defining trait, rooted in her past achievements; after a middle school team competition where she performed well, she came to declare herself the best first-year player in the nation, although she excludes players with extraordinary, "monster-class" abilities from this self-assessment. This confidence, however, coexists with a strong sense of responsibility. She is headstrong and proactive, demonstrating leadership qualities even as an underclassman among older teammates. Despite her self-assured demeanor, she is acutely aware of her own failures and feels deep remorse when she underperforms, believing she has let her team down.

Izumi's primary motivation is tied to her ambition to prove her strength on a national stage. She enters the national tournament determined to succeed, particularly driven by her memory of a past loss to Nodoka Haramura in middle school. In the story, her role is that of a capable but non-supernatural player thrust into an environment where many opponents possess unique, almost inexplicable abilities. Her key relationships are with her teammates at Senriyama. She is particularly close with the third-year student Toki Onjōji, whose health issues concern the entire team. Izumi also looks up to her senior, Sera Eguchi, from whom she seeks and receives comfort after a crushing defeat. She often interacts with the analytical Hiroko Funakubo, who provides data and strategic insights.

The national tournament arc is the central stage for Izumi's development. Her journey is marked by a stark contrast in performance between matches. Initially, she performs solidly, using her stable play to predict her opponents' moves, such as against Yuu Matsumi. However, her confidence is shattered in a later match against Shiraitodai High School. Faced with the powerful Sumire Hirose, whose "piercing shots" are a supernatural ability, Izumi’s conventional defensive techniques prove useless. She loses a massive number of points, fails to win a single hand, and ends the match in a state of profound disappointment and embarrassment, realizing she is the only first-year player at the table. This humbling experience serves as a crucial turning point. After the match, her teammates, particularly Sera and Hiroko, help her process the loss. Hiroko explains that the national tournament has many exceptional first-year players, a category Izumi herself does not fall into, forcing her to confront the gap between her self-perception and reality. This moment of crisis hints at a shift from pure bravado to a more mature understanding of her own limitations and capabilities.

As a player, Izumi does not possess any supernatural or "special" abilities. Her talent lies in her stable, analytical approach to the game, which makes her excellent at protecting her points against ordinary opponents. She can read the flow of a hand and predict dangerous situations to a certain degree. However, this conventional strength becomes her critical weakness when faced with players who have extraordinary abilities that defy normal mahjong logic, as she cannot defend against what she cannot predict.