Rouka Numachi is the former basketball rival and supposed arch-nemesis of Suruga Kanbaru. Though she once shared an affinity for basketball akin to Suruga, albeit as a defensive player, she would go on to sustain a leg injury that would end her endeavor in sports. The two had a falling out in middle school due to Rouka's injuries causing a rift between the two.
Rouka is characterized by the concept of "schadenfreude"; in which she derives pleasure from the misfortune of others. A self-proclaimed collector of misfortunes, she enjoys listening to people's problems whilst offering empty promises of helping them out. Turning this form of counseling into a non-profit service, Rouka began to operate under the alias called "Lord Devil"; in which she promises her customers that she can solve anyone's problems with absolute certainty.
Contrasting Suruga's virtuous nature, her desire for collecting misfortunes is one of malice. Though she genuinely sympathizes towards her customers' issues, her actions are maliciously self-beneficial and acted out with impartiality towards her customers. She justifies her sense of "evil" as non-harmful, as it not only benefits her state of mind while also allowing her customers to solve their own problems by simply letting the time pass. She carries a crutch due to her injury, which she described as allowing her customers to open up more easily with her.
In Hanamonogatari, it is revealed that Rouka, through her business acquaintance Kaiki Deishuu, had discovered the nature of apparitions/aberrations. When Suruga first confronted Rouka about her counseling activities as "Lord Devil", it is later revealed that Rouka had stolen Suruga's supposed "monkey's paw" (a part of the "Rainy Devil" apparition) during the encounter; in which she transfused it onto her own left arm.
Rouka had been collecting the various body parts of the "Rainy Devil" apparition since she viewed its problem-solving abilities as a competitor and threat to her business. During her second confrontation with Suruga, Rouka elaborated on her motives for collecting misfortunes and "Rainy Devil" parts, even revealing that her current leg injury is a ruse, due to the transfused leg of the "Rainy Devil" negating her past disability.
Upon further investigation, Suruga discovered that Rouka already died before she even graduated to high school; having committed suicide out of grief over her injuries. Suruga had actually been meeting Rouka's ghost, in which the latter had forgotten of her own death and had not yet ascended. During their final confrontation, Suruga presented the head part of the "Rainy Devil", not before challenging Rouka in a one-on-one basketball match with the intention of exorcising Rouka's ghost. Suruga proposed the match under the condition that if she were to win, Rouka must give up her pursuit of collecting misfortunes and "Rainy Devil" parts. Suruga won the match and the two expressed contentment over it, showing admiration for each other. Having forgotten what she had lost in life, Rouka mused how she was truly happy to have finally lost to Suruga before finally ascending; leaving behind her collection of "Rainy Devil" body parts.
