Momiji Ooka is a second-year student at Kyoto Senshin High School and a prominent figure in the world of competitive karuta, the traditional Japanese card game based on the Hyakunin Isshu anthology. She is the heir apparent to the title of Queen of Karuta, having already become the national high school champion. Her family is extremely wealthy and influential; her grandfather is a former Prime Minister of Japan, and the Ooka Group is a major corporate entity. This background gives her access to a personal butler, Muga Iori, a former public security police officer who is fiercely loyal to her.

Momiji first appears during the events in Kyoto and Osaka, where she introduces herself to Heiji Hattori as his future wife. This claim stems from a childhood encounter at a karuta tournament: after losing to Heiji, she misheard his words of encouragement, mistaking the phrase for stronger (tsuyome) for bride (yome). Although the misunderstanding was later clarified, Momiji continues to pursue Heiji with persistent, good-natured determination, viewing him as her destined partner. This puts her in direct romantic rivalry with Kazuha Toyama, whom she calls Kazu-chan. Their relationship is competitive and teasing, but they also show mutual respect and have worked together to help solve cases.

Momiji is shrewd, observant, and possesses exceptional deductive abilities. Her keen hearing and sharp eye for detail allow her to pick up crucial clues that others miss, such as identifying a suspect from a phone call sound or noticing the odd behavior of a dropped object. She often assists Heiji and Conan Edogawa in solving mysteries, relaying hints through her butler or deciphering codes herself. Beyond detective work, she is skilled in using her knowledge of Kyoto’s geography and traditional culture to interpret hidden messages. Despite her generally cheerful and slightly vain demeanor, she is also empathetic and brave, willing to put herself in danger to protect friends like Kazuha during hostage situations.

Throughout her appearances in both the film and subsequent manga arcs, Momiji’s motivations remain centered on winning Heiji’s heart while also pursuing excellence in karuta and using her intellect to aid in investigations. She sponsors events, such as a detective play, and influences key plot points that push Heiji toward a confession to Kazuha, even as she herself grapples with her own feelings. Her character provides a blend of romantic tension, comedic rivalry, and genuine problem-solving capability within the larger Detective Conan world.