Oji Karasuma is a second-year student in Class 2-C at Yagami High School and one of the central figures of the story, serving as the object of Tenma Tsukamoto's affection. He is a quiet, elusive young man with short black hair cut in a bowl style and black eyes, born on October 28 and standing about 175 centimeters tall. In class he is most often seen silently gazing out the window, and his classmates generally regard him as a strange person who appears to exist in his own world.

Karasuma's most defining traits are his expressionless face and his extreme taciturnity. He rarely speaks, shows almost no emotion, and is difficult to read even to people who know him well. This seeming detachment is paired with a set of peculiar habits, the best known being his practice of dressing as a kappa on rainy days, a visual pun on the Japanese word for raincoat. His favorite food is curry, and his attachment to it is so strong that when asked to choose between curry and the girl who likes him, he chooses curry without hesitation. Despite his aloof exterior, he is not truly indifferent to others. He shows quiet concern for the people around him, particularly Tenma, and he sometimes behaves like a mentor, dropping small hints or challenges that encourage her to discover her own strengths, such as when he gives her a turtle.

Karasuma's role in the story grows out of the one-sided romance that Tenma develops for him. She struggles to confess her feelings because he appears listless and barely engaged with the world around him. A long, unsigned love scroll from Tenma, which ends with the plea for him not to go, convinces him to postpone his planned move to the United States at the end of the school year, and this decision keeps the central love triangle in motion. His quiet presence also places him in an indirect rivalry with Kenji Harima, who loves Tenma and repeatedly tries and fails to win her attention, while Karasuma remains her focus. Karasuma's relationship with Tenma develops gradually through her persistent efforts, and over time he begins to respond to her more warmly, though he still shows little outward emotion.

A deeper layer of his character is revealed through his secret life. Karasuma is a professional manga artist who publishes under the pen name Nijou Jou, a career that he keeps hidden from his classmates. He also plays electric guitar in a school band, and in the anime he is shown as a soccer player who wins a cup with the national team. His talents extend to physical ability as well. Although it is never directly stated, he is remarkably skilled at dodging flying objects, including Harima's Hurricane Kick and Tenma's arrow-shaped letters, which suggests a sharp awareness that contradicts his absent-minded appearance.

Karasuma's development is shaped by a serious illness that will gradually erase all of his memories after he goes to America. Because he knows that he will eventually forget everything, he deliberately hides his feelings for Tenma in order to spare her from future pain. When Tenma finally works up the courage to confess, he does reciprocate her feelings, but shortly afterward he loses his memory. This loss changes the direction of Tenma's life, giving her a new sense of purpose as she focuses on her studies and on being there for him. His arc ultimately transforms him from a distant, unreadable classmate into a person whose quiet restraint comes from genuine care and a desire to protect the girl he loves from the consequences of his condition.