Kaori Orimoto is a former middle school classmate of Hachiman Hikigaya, the protagonist. She attends Kaihin Comprehensive High School, a different school from Hachiman and the main cast of Sobu High School. Her most significant role in the story is tied to Hachiman's past, as she is the girl who rejected his romantic confession during their middle school years. This event became a formative trauma for Hachiman, deeply influencing his cynical worldview and his distrust of sociable, friendly girls.

Physically, Kaori has a distinctive short, bobbed hairstyle. While described with black hair in the original source material, she is typically depicted with light brown hair in the anime adaptation, paired with brown eyes. Her outward personality is a core part of her character. She is a very energetic, open, and sociable girl who adopts a cheerful "big sister" persona. She has a habit of approaching anyone without hesitation and often uses a lighthearted catchphrase that translates to "That happens!". This easygoing and friendly demeanor, however, can sometimes lack sensitivity.

At the start of the series, Kaori functions primarily as a figure from Hachiman's dark past. Her offhanded way of discussing his confession with friends after the fact led to rumors that painted him in a humiliating light, a betrayal that helped forge his antisocial tendencies. She is first shown on-screen during a chance encounter with Hachiman at a donut shop, where she cheerfully reintroduces herself to him and his acquaintance, Haruno Yukinoshita, openly bringing up his past confession without apparent malice. This interaction highlights a key aspect of her personality: a lack of awareness regarding the weight her words can carry.

As the story progresses, particularly during a joint planning committee between Sobu and Kaihin High Schools, Kaori takes on a more direct role. Her key relationships develop from this point. Her dynamic with Hachiman slowly shifts from one defined by an awkward past to a more normal, if still somewhat distant, acquaintance. She teases him gently but also begins to show a more thoughtful side, acknowledging that he has changed and is not as dull as she once thought. She also interacts briefly with Hayato Hayama, a popular boy from Sobu High, exchanging contact information with him, which later leads to an uncomfortable double date. Through these interactions, especially with the perceptive Hayama, she is subtly made aware of her own social flaws, including her tendency to be thoughtless.

Kaori experiences a degree of character development that adds depth to her role. While initially presented as just the source of Hachiman's trauma, she later demonstrates a capacity for growth and self-reflection. She admits to her past mistakes and makes a genuine, if awkward, effort to be a better friend to Hachiman. Furthermore, she reveals a sharp and perceptive side, correctly identifying the complex and unusual dynamics within the Service Club, the main group of characters. Unlike many who are fooled by Hachiman’s self-sacrificing schemes, she is able to see the underlying logic and the true feelings between the members.

In terms of notable abilities and hobbies, Kaori is an accomplished cyclist who owns a road bike, a hobby she is passionate about and often tries to share with others. She also has an interest in photography. These traits paint a picture of an active and inquisitive person. While she is skilled at breaking the ice and creating a friendly atmosphere in public settings, she notably struggles to form deeper personal connections with the female members of the Service Club, finding that her usual sociable tactics do not work to build a real friendship with them. Her role ultimately evolves from a symbol of past failure to a more rounded character who demonstrates that first impressions are not always final, and that people have the capacity to change and recognize their own shortcomings.