Class 1-B Tomoko is a lively cheerful girl, she seems confident on the outside and a bit hard-headed, but she is actually an anxious girl with a constant fear of being judged for her past actions. Ohinata was a first year at Kamiyama High School when the members of the Classics Club entered their second year. Ohinata became interested in the Classics Club at a New Recruit Fair, where Houtarou Oreki and Eru Chitanda were trying to recruit new members of the Classics Club. Ohinata became interested when she overheard Oreki and Chitanda trying to solve a mystery about a mysterious table in the possession of the Confectionery Club at the fair. After hearing Oreki solving the mystery, Ohinata was convinced to join the Classics Club.

Spoiler

Ohinata eventually spent a lot of time on the club, among others joining the other members to celebrate Houtarou's birthday party at his house, and later inviting the other Classics Club members to test out a cafe opened by one of Ohinata's relatives. However, while at the cafe, a magazine was brought out that reminded Ohinata of a dark event in her past. In her middle school years, she had been on a country-wide trip with her friend, following a band from Sendai to Fukuoka. Her friend had paid for the trip, unbeknown to Ohinata using money she had swindled out of her own uncle. When Ohinata found out, she felt guilty because her friend had stolen the money for Ohinata's sake, at least that's how Ohinata felt, and she felt relieved when the friend had too few points in her test to attend Kamiyama High School alongside her.

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After the incident in the cafe, Ohinata was afraid Chitanda would find out about her past, which led her to quit the Classics Club the day before the Hoshigaya Cup race, after 41 days of membership. Her leave from the club was unexpected and left Chitanda feeling guilty and the other members puzzled. During the course of Volume 5, Oreki fits the pieces of the puzzle together to figure out the truth behind Ohinata, finally getting the full picture when inviting Ohinata to a dango shop in the final chapter. While Ohinata is relieved to get the truth off her chest, she elects to not rejoin the Classics Club again.

Spoiler

Ohinata also appears in the short story Tiger and the Crab, or the Murder of Oreki Houtarou, in which she brings in a magazine containing an embarrassing book report made by Oreki in middle school. Ohinata also completed the embarrassment of Oreki when she was the first to realize Oreki had actually hidden clues in the report that he really liked book reports.

(Source: Hyouka Wiki)