Kuro Matsumi is a second-year student at Achiga Girls' Academy in Nara Prefecture and a key member of the school's mahjong club. Before the club was temporarily disbanded, she had been its strongest player. During the period of inactivity she continued to care for the clubroom, keeping it clean in the hope that someone would eventually return and revive the team. She is the younger sister of Yuu Matsumi, who is also a member of the Achiga team.

Kuro's personality is marked by a gentle and upbeat disposition. She carries a sincere kindness and a strong sense of responsibility toward her teammates, often placing the group's well-being above her own individual performance. Her dedication to the club and to mahjong itself is rooted in a genuine love of the game and a desire to share it with friends, especially after experiencing the loss of the original club. This loyalty makes her a grounding presence within the team.

Her primary motivation at the national tournament is to help Achiga succeed alongside her friends, a goal that includes the chance to reunite with childhood acquaintance Nodoka Haramura, who now plays for Kiyosumi High. The opportunity to compete on a national stage and test herself against the country's strongest players further fuels her drive.

Within the story of the national competition, Kuro plays a crucial role as Achiga's opening player in the semifinals, where she faces an extraordinary gathering of opponents, including the reigning champion Teru Miyanaga of Shiraitodai. This match becomes a defining moment for her. Initially, her signature ability is exploited by observant rivals, causing Achiga to fall behind. Yet rather than simply being overwhelmed, Kuro responds by making a deeply personal and risky break from her own established playstyle, a move that surprises even a national-level monster like Teru and helps keep her team in contention.

Kuro's key relationships center on her bond with her older sister Yuu, whose quieter approach to the game contrasts with Kuro's own strengths. She shares a tight friendship with Shizuno Takakamo, the spirited captain who pushed for the club's revival, and with Ako Atarashi, a fellow childhood friend. These relationships reinforce the teamwork and emotional support that define Achiga's underdog spirit.

Her development is most visible in how she confronts the limitation attached to her power. When opponents successfully neutralize her ability to draw dora, she is forced to play without her usual advantage. In the intense semifinal match, she chooses to discard a dora tile, an act that severs her connection to those bonus tiles for many subsequent games but frees her from a predictable pattern. This decision marks a turning point in her growth, showing her willingness to sacrifice her personal blessing for the sake of the team and to rediscover the fundamental joy of mahjong beyond reliance on a single ability.

Notably, Kuro possesses a supernatural gift that makes her a formidable scorer: an almost gravitational attraction of dora tiles into her hand. This ability ensures she consistently assembles high-value hands and has earned her the nickname "The Dragon Lord." However, it carries a strict condition. Should she ever discard a dora, she loses the blessing entirely and will not draw any dora at all for a prolonged stretch of games. This constraint shapes much of her strategic and emotional arc, making her a player defined by both a remarkable blessing and a difficult personal choice.