Kasen Tosu, wife of Tetsuo Tosu and mother of Reika Tosu, is 41 years old. She originates from wealth via her family's "Kimono no Tosu" shop, though now estranged from her parents. Kasen and Tetsuo are blood-related cousins. Her past includes involvement in a rural cult led by her father, an association she ultimately escaped. Despite this separation, Kasen and her mother preserved their practice of communicating with spirits of deceased cult members, a skill they later harnessed for deception when advantageous.

Kasen exhibits profound loyalty to her family. Discovering her husband at the scene of Nobuto Matori's murder, she immediately aided Tetsuo in covering up the crime. She displayed remarkable calm and strategic capability under pressure, disposing of Nobuto's remains and countering yakuza surveillance. Her actions encompassed evidence concealment, alibi fabrication, and participation in schemes to mislead the pursuing crime syndicate, marking a stark departure from her prior life as a housewife and showcasing her adaptability under threat.

Physically, Kasen has brown or black hair and brown eyes. As the story unfolds, she confronts escalating dangers, including direct encounters with yakuza investigating Nobuto's disappearance. While their daughter Reika remained minimally involved initially, Kasen prioritized shielding her family's stability. Her resilience is further tested and demonstrated by her pregnancy and the subsequent birth of another son later in the timeline.

Her name, Kasen (歌仙), reflects meanings like "poets" or "great poet," while Tosu (鳥栖) combines "bird" and "nest." Her character evolves from a supportive spouse into an active accomplice in criminal activities, leveraging her background in manipulation and spiritual pretense to outmaneuver adversaries. Kasen remains a consistent presence across all adaptations—manga, anime, live-action television drama, and film—without significant deviations in her core traits or narrative role.