Mairu Shirouzu is a third-year student at Shindouji Girls High School, where she serves as the captain of the mahjong club and has been the team`s ace player for three consecutive years. She has been a close companion of her teammate Himeko Tsuruta since their middle school days, and this long-standing bond forms the core of her character and her unique approach to the game.

In terms of personality, Mairu is typically calm, collected, and pragmatic. She possesses a strong sense of leadership and is deeply considerate of her teammates, often displaying an astute understanding of their individual thought processes and the pressures they face. For example, she offered words of reassurance to Kirame Hanada following a challenging vanguard match, and she accurately deduced the strategic mindset of her teammate Yoshiko Yasukouchi. While she can come across as strict or serious, her interactions with the team reveal a more relaxed and sometimes teasing side, especially when she is trying to encourage or motivate them.

Mairus primary motivation is to lead Shindouji to victory in the national high school mahjong tournament. Her role in the story is pivotal, as she is the strategist and emotional anchor for her team. She is responsible for not only achieving her own wins but also for setting up conditions that allow Himeko to succeed in the later captains match. Her tactical decisions are always made with the overall team score and her teammates` capabilities in mind, demonstrating a forward-thinking approach to the competition.

The most significant relationship in Mairus life is with Himeko Tsuruta. Their connection is exceptionally strong, to the point where it directly influences their mahjong abilities. They are often seen together, supporting one another, and their synergy is the cornerstone of Shindoujis strategy. Mairu also maintains respectful and thoughtful relationships with the other members of her club, including Kirame Hanada, Yoshiko Yasukouchi, and Hitomi Ezaki, recognizing their strengths and roles within the team.

Throughout the story, Mairu demonstrates development by adapting her traditionally safe and calculated style of play. Known for going for quick hands to build points steadily, she shows growth when the situation demands greater risk. In high-pressure matches, she abandons her usual defensive caution, chasing larger, more difficult hands to make significant dents in the point difference when her team is behind. This willingness to step outside her comfort zone highlights her determination and her ability to rise to the occasion as a true ace.

Mairus most notable ability is called Reservation. This is a power that can only be used in tandem with Himeko Tsuruta. At the start of a hand, Mairu looks at her initial tiles and imposes a self-restriction by setting a minimum han value, such as two, three, or four han, that she must achieve to win the hand. She then places her tiles face-down before reopening them, committing to her chosen goal. If Mairu successfully wins the hand with that exact number of han or more, Himeko automatically wins the same round in her own match with a point value double the amount Mairu originally set. However, if Mairu fails to win her hand after setting the restriction, Himeko will almost certainly be unable to win her corresponding hand; if she does win by chance, it will likely be for the minimum value of one han. Conversely, if Mairu chooses not to activate her ability, the results of their hands remain unrelated to each other. This ability places immense pressure on Mairu to succeed, as her failure directly harms her partners chances, while her success can lead to a massive swing in the team`s overall score.