Komachi Hikigaya is Hachiman's younger sister. She is also a member of her school's student council.
Komachi appears to be a normal middle school girl of average height. She is usually seen wearing her middle school uniform. She has short-medium length black hair. and just like Hachiman, she has a strand of hair standing straight from her head (ahoge). In the light novel, it was mentioned that Komachi is flat chested even for a middle school student, but this is overshadowed by her outward personality.
Unlike her brother, Komachi has a cheerful, lively and forgetful personality which was shown when she revealed she ate the sweets her brother was meant to get from Yui and not telling him about it. Hachiman considers his sister to be an idiot and fool, at least when it comes to education. Komachi often borrows from his old projects and asks him to help her study.
She enjoys teasing her brother as she often tries to set up reasons to get him alone with Yui or Yukino. She is sly in handling things like setting her brother up in a date like situations or tries to get things in her favor. Unknown to her, Hachiman is actually well aware of her tactics and tries to go around it as Komachi gets frustrated with him, thinking he is unaware or because of his lack interest in it.
According to Hachiman, she's got the Big Three: cuteness, prettiness, and good looks. Komachi is known for her charming personality which seems to attract everyone. Because of this, she mentioned to be a popular girl in her school despite the fact that Hachiman is her older brother (who was unpopular and not very well-known). Although mentioned to be popular she is only shown to hang out with Taishi.
She is a kind and caring person with an easygoing personality. This can be seen when she requested help from the service club for Taishi and also in Chiba summer camp where she helped the elementary school students.
She can also be serious and cold to people. In one incident, she refused to speak to her father for reasons unknown, which reduced her father to tears even to the point of him begging her to talk to him again. She also refused to talk to her brother Hachiman when he was rude to her when she tried to help him until he consulted her for advice and apologized.
It is mentioned in the light novel that she often spends all weekend at home with her brother. Being aware of both the merits and demerits of being social and loner, she chooses the best options that favors her and also chooses when, where, how and to whom she must socialize.
She has very good cooking skills as she is usually the one that creates meals for herself and Hachiman. This can be seen in OVA. She knows what Hachiman likes and dislikes when it comes to food and people.
She seems rather perceptive of her brother as she is able to notice that Hachiman had done another social suicide even though she was not present at the time. She is good at taking care of Hachiman and knows most of his needs as quoted by Yukino.
- "Is my mouth jammed?" (referring to jam she has on her face). - Komachi to Hachiman
- "Komachi -teki ni point ga takai." - Komachi to Hachiman
- "Oni-chan,What are you saying?, You dumbass! Nitwit!, Hachiman!" - Komachi frustrated with Hachiman's lack of interest in romance
- She was the Point of Character for Track 6 of drama bonus C.D titled Tatoeba Konna Birthday Song (たとえばこんなバースデーソング, "Birthday Song for you") of Volume 3.
- On Wataru Watari's Twitter Profile, his profile picture is the visual novel version of Komachi Hikigaya. It may be an indication that she is Wataru Watari's favorite character in the series.