Kasumi Iwato is a third-year student at Eisui Girls High School in Kagoshima Prefecture, where she serves as the president of the school's mahjong club. As a member of a family of shrine maidens who belong to a branch family of the maiden lineage, her life and abilities are deeply intertwined with spiritual traditions. Her position on the team is that of captain, a role that carries the responsibility of closing out matches in the national tournament.

In terms of personality, Kasumi is consistently depicted as a very kind and polite girl. Her demeanor is refined and calm, and she maintains a close, familial bond with her teammates. This is especially evident in her relationship with her cousin, Komaki Jindai; unlike others who refer to Komaki with the formal title of Princess, Kasumi affectionately calls her "Komaki-chan". A flashback reveals that her determination to join the main family of maidens was motivated by her childhood friendship with another teammate, Hatsumi Usuzumi, whom she has known since the age of seven.

Kasumi's motivations are largely rooted in her duty as a shrine maiden and her role on the team. Her primary purpose is to protect Komaki, who acts as a vessel for nine goddess spirits. Because Komaki cannot contain a particularly rare and malevolent spirit, Kasumi, as the closest blood relative, volunteered or was designated to become its vessel, a role she has served since she was eight years old. Her role in the story comes to the forefront during the national tournament finals. Known for a highly defensive and stable playing style, Kasumi’s record shows she never deals into an opponent’s hand, only losing points through other players winning by self-draw (tsumo). Her primary narrative function in the finals is to face the protagonist, Saki Miyanaga, where she is forced to abandon her defensive nature and unleash her powerful but risky ability to secure a lead.

Her most significant relationship is with Komaki Jindai, to whom she is related by blood and spiritual servitude. She also shares a deep history with Hatsumi Usuzumi from their childhood. Other teammates, such as Kyouko Suehara, recognize Kasumi's talent, with one observer calling her the most dangerous player on the Eisui team. Her family connections also include another cousin, Akise Iwato.

Kasumi's character undergoes notable development during her match in the national tournament. Initially, she plays cautiously, analyzing her opponents to maintain the point lead provided by her earlier teammates. However, when she begins to fall behind, she makes the conscious decision to stop playing defensively and invoke her ability, despite the drawbacks. This act transforms her into an aggressive, high-scoring player, creating a direct contrast to her usual style. After gaining the lead, she is left momentarily uncertain, as her ability prevents her from reverting to the defensive play she prefers.

Kasumi Iwato's notable ability is a direct manifestation of the goddess spirit she hosts. When she activates this power, she changes the flow of the tiles so that she draws exclusively from a single suit plus honor tiles. At the same time, this prevents her opponents from drawing any tiles of that specific suit, effectively sealing their ability to use those tiles. This power creates a situation where she can easily build "flush" hands (like a half-flush or full flush) for massive point gains. However, this ability comes with a severe limitation: once activated, she cannot turn it off or switch back to her normal playing style until she has undergone a purification ritual performed by her teammates, which cannot be done during a match.