Momiji Ooka is a second-year student from Kyoto High School and a champion of Hyakunin Isshu karuta. Her family is likely wealthy since she has a butler. Momiji claims Heiji Hattori proposed her when they were little and thus believes that they will be romantically involved in the future.
She was once part of an elite karuta association called Nagoro-kai. She was the greatest student of the group's founder, Nagoro Shikao. However, after the group was disbanded she joined another association called the Satsuki-kai.
Momiji appears to be an extremely competent detective, as she saw through a trick merely based on a sound she heard in a phone call with her butler, Muga Iori, before Conan, Heiji or Bourbon did. Later, Muga says that her deduction was great as usual, implying that she regularly solves cases and also says that Heiji's great deductive skill was to be expected from the person that Momiji chose as her partner.
Momiji is involved in the plot of Movie 21 which will center around Heiji and Kazuha's romance. She is shown crying in the movie artwork. The trailer of the movie shows her in an uta-karuta tournament and introducing herself as "Heiji's fiancé", plus there's a picture of her and Heiji's younger selves making a pinky swear.
(Source: Detective Conan Wiki)