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.