Orimoto Kaori was mentioned in Volume 1 but not introduced until Volume 8 of the Light Novel and Season 2, Episode 3.
It is described that one of the main reasons why Hachiman is reluctant to make friends is because of his past experience with Kaori Orimoto. As middle school classmates, Hachiman confessed his feelings towards Kaori and consequently had his feelings rejected. Even though many incidents are implied of Hachiman's romantic interest towards and poor treatment from various girls in past, Kaori Orimoto is the only girl to whom he openly confessed his feelings to. Despite her being the only person present at the time of confession, the whole school came to know about Hachiman's confession and mocked him for it. It may be because she put on air about being confessed.
It is explained how Hachiman got her contacts in Volume 2 Chapter 1. He recalls how innocent he was not to notice the pity of receiving her number until she stopped replying his text after two weeks. She only exchange her contacts after witnessing every girl avoiding exchanging contacts with Hachiman. In Light novel, Hachiman overheard Kaori talking to her friend about how irritating it was to be texted by Hachiman Kaori grossed at the thought of her friend pointing out the possibility of Hachiman having feeling for her, without knowing Hachiman heard their conversation.
According to Hachiman, he got her mail address out of pity. He would text her for foolish reasons and anxiously wait throughout the night sleeplessly for her replies that may or may not come. But Kaori explains to Haruno that they never talked before, indicating that she didn't remember or acknowledge their conversation with Hachiman. This may be due to the minimum conversation with her, on Hachiman part or large number of social contacts of kaori with others.
It was mentioned that Kaori appeared to be friendlier with Hachiman than other people, but it was ruined after Hachiman's confession. As stated in first season, she never spoke to him again. Though she is friendlier/social with Hachiman, she tends to look down on him in the same way others do.
Hachiman meets Kaori again coincidentally while at a cafe with Haruno. Kaori recognized him and told Haruno about Hachiman's confession which Hachiman tries brush off as a mistake from his younger self. She stated her opinion of Hachiman not capable of scoring with Haruno. She also showed interest in getting to know Hayato through Hachiman & Haruno. Haruno got Hayato to come to the cafe where Kaori completely ignored Hachiman and focused solely on Hayato. Kaori and her friend also exchanged contacts with Hayato. But no exchange was mentioned between Hachiman and the girls.
A double date was arranged with Hayato, Hachiman, Kaori, and Chika, however Hachiman refuses to go and claimed to have not been invited. Hayato forced Hachiman to come along by contacting Haruno for help.Haruno tried to negotiated with logic but threatens to drag him from his house to which Hachiman comply with her proposal of walking along with them.
The date plan was to watch a movie, go shopping and get dinner. The girls showed no interest in Hachiman and talked almost solely to Hayato.They excitedly greeted Hayato and begin their date by talking to him and only him while walking along with him while Hachiman followed them two steps back.
In the theater, Hayama is seated between the girls with Hachiman seated next to Kaori. Kaori also expressed her surprise to Hachiman about watching a movie with him. She also adds with a held up laughter, that their middle school friends would freak out if they witnessed it.
Kaori and her friend mostly belittled Hachiman while praising Hayato in comparison and joking about Hachiman's past. For dinner they ridicule Hachiman's choice of Saize, and openly praise Hayato for his selection that they end up going to. The date ended in disaster as after dinner Hayato coldly told the girls he was tired of their boorish behaviour and to not judge people by their appearance.
He mentions that Hachiman has befriended girls who are far better than both of them, pointing to Yukino and Yui who arrived to meet Hayato, unbeknownst to everyone. This caused them to be ashamed of their actions and took their leave immediately. As they are leaving, Kaori took a look at both Yui and Yukino, Kaori assumed that one of them was Hachiman's girlfriend.
Christmas collaboration event
Kaori once again runs into Hachiman, this time at the Christmas collaboration event held by both of their schools. First she was surprised by Hachiman presence, as she didn't think he was a type of guy to participate or volunteer and asked if he was with anyone and anticipated few peers but he answered negatively. Kaori maintained her casual and friendly appearance to Hachiman, but was more considerate with her words than before. It was mentioned that she volunteered to help her friend on the planning committee. She gave her approval enthusiastically for every idea, no matter what it was.
Sometime later, she meets Iroha and Hachiman in the rain. She is nervous in meeting him as she stated that she knew him middle school but doesn't elaborate on it even denying they were close.
Kaori is next seen at the meeting, this time praising ideas and seems to be surprised by Hachiman's way of handling the planning committee and wonders if he was in the student council back in middle school. She later meets with him where she inquires about his relationship with Iroha and the other two girls whom she met at the café both of whom he refers to as "just club mates". She is surprised that he was in a club but laughed at its name, though apologized as she had no understanding of its purpose.
After the Christmas event is set into motion by Hachiman and Yukino, Kaori met up with Hachiman once again. She bought him a drink and explained her misjudgment about him in the past. She expressed her praise of Hachiman and states there was more to him than she realized while they cannot engage in a romantic relationship as he still wasn't boyfriend material in her opinion, though he voices that he is no longer interested in her. She surprises him by smiling and stating they could be friends stating that he is interesting/funny to her. She soon invites Hachiman to their Junior high reunion, to which Hachiman bluntly turned down as Kaori laughs thinking it was a joke to Hachiman's irritation.
In Light novel volume, this scene proceeds as both Kaori and Hachiman cycle their way to their respective house. It was clear that Kaori didn't remember much, including the names of Yukino and Yui even after the end of the event, as she address them as the girls close to Hachiman. Kaori was surprised that Hachiman lived in the same region as before, as she didn't saw him there after graduation. Hachiman internally blames her for it, for he was trying to avoid all from his past especially her and stated indoors, but found it surprising to know that she remembered and knows where he lived.
Pre-Valentine's day collaboration event of cooking class
Kaori was also a participant in making valentine chocolate event. Kaori sounded as if she anticipated him to be there and greeted him energetically. She also gave short greet to Yui and Yukino before joining her schoolmates. Later, Kaori borrowed a mold for the chocolate from Yukino through Hachiman. She soon pondered on something and asked him if she ever gave any Valentines chocolate to Hachiman in the past, attracting curious (and jealous) looks from Yukino, Yui, Iroha, Saki and surprising Tamanawa as well (who possibly has a crush on Kaori). When Hachiman replies that she didn't, she promises to give Hachiman some this time, which she does at the end of the event in a paper plate.
Hachiman recalled how Kaori was a type of girl who gives anyone Valentine chocolate as long as she talked to them before and saddened by the fact that he didn't even fall under that category in the past, He wondered how he took it back then.