It is mentioned that she goes to the same middle school that Hachiman went to. She also aims for Sobu High because her brother is there. But her reason seems to be rude. She says that she will look like a nice girl in everyone's eyes in comparison to her brother. She was treated like an angel in middle school for having to "put up with" her brother.
Komachi innocently revealed that Yui came to visit Hachiman as a thanks for saving her dog with sweets but couldn't meet him because he was "sleeping" at the time. Komachi mentioned that she (Yui) would thank him at school personally since they go to the same school and handed Komachi the sweets which she ate herself and forgot to inform Hachiman (until they met again in Episode 5).
One time Komachi ran away from home once because she hated returning to an empty house. It was Hachiman who found her and he has been returning home before her ever since. However, in light novel it was mentioned to be a lie told by Komachi to make Saki understand her brother's concern over Saki.
In drama CD, it was mentioned that in middle school, Hachiman gave a present to the girl whom he liked on her birthday, by staying up all night compiling a bunch of anime songs to recommend. His present was accepted, but tragedy occurred the next day. At lunch time, in the PA system of the school, the broadcast committee chose his song by saying : ‘Aaaand the next song was requested by Class 2C’s Otagaya Hachiman-kun (snicker), a love song (snicker) for Yamashita-san!’.
After this incident, the name Otagaya passed on even after his graduation, to which Komachi expressed her difficulty in pretending that Otagaya was not her brother.
She used an underhanded method in securing her victory in student council membership. She openly announced her wish to everyone in class about joining the student council, thus prevented others from contesting in concern for their social standing and to avoid bad rumors about them of snatching away Komachi's dream.
In Episode 5, Komachi get the Service Club to help her friend Taishi, with solving his sister's problem. She also reveals to Hachiman that Yui is the owner of the dog he saved.
In Episode 7, Komachi tricked Hachiman into going with her to Chiba summer camp for a summer Service club activity.
Komachi went to Sobu High school's cultural festival to explore the place.
In Season 2 Episode 3, After the field trip, Komachi asked about Hachiman's odd behaviour with the intention to help him. Hachiman was unable explain and rudely stated Komachi should mind her business. Komachi then angrily storms off and refuses to speak to him until later in the series when he apologizes.
They finally reconcile after Hachiman apologized and asked for her help. Komachi realizes her brothers issue in wanting to protect his friends. Komachi makes a personal request to Hachiman to stop Yukino and Yui from becoming student council president, in order to save the service club from disappearing which Hachiman succeeds in.
In the series she is shown to study hard for the entrance exam of Sobu High. In Episode 13 of Season 2, she was going to take the entrance exam of Sobu High School.