Matsuri Touba, the 18-year-old cousin of the four Ryudo brothers, resides with them and manages key household duties like cooking and cleaning. Her presence offers a stabilizing feminine influence in their home. All four brothers deeply cherish Matsuri and fiercely protect her. She reciprocates their care, showing particular attentiveness to the eldest brother, Hajime; their interactions carry an undercurrent of mutual romantic tension, constrained by moral boundaries. Matsuri enjoys the company of Tsuzuku while adopting a nurturing, maternal role toward the younger brothers, Owaru and Amaru, guiding their growth.

She is the daughter of Seichiro Toba, the self-serving head of education at Kyowa College, and Saeko Toba, who is the Ryudo brothers' aunt. The brothers hold Saeko in high regard, a stark contrast to their disdain for Seichiro. Matsuri herself exhibits profound contempt for her father, rooted in his unethical conduct—specifically exploiting his position to strip Hajime of his teaching directorship. Her disdain reflects awareness of his morally questionable motives, including the implication that he married Saeko primarily to gain control of the college. This familial conflict places Matsuri in opposition to her father's ambitions, reinforcing her solidarity with the Ryudo household.

Within the narrative, Matsuri functions as the household's emotional anchor. She fosters normalcy amidst the brothers' supernatural conflicts through steadfast domestic support and unwavering loyalty. The deep mutual protectiveness between her and the Ryudo brothers underscores her integral role within their familial unit.