Both of Komachi's parents are unnamed and are working in a corporate company, so they are always either late or outside the house, thus rarely shown in the series. It was mentioned that they never celebrated Hachiman's birthday, but they went out shopping during Komachi's birthday to buy dress and to celebrate showing they favor her over him.
Her father is mentioned to care for Komachi's well being and strives for her love and acknowledgement. When she was upset with him and refused to talk to him he wept. Her father threatens any boy that comes close to Komachi even Hachiman.
Her mother was mentioned in episode 7 of season 2 where she asked Hachiman, through Komachi, to buy chicken and cake for their Christmas dinner. In episode 13 of season 2 Hachiman mentioned that Komachi and their mother are at "each others throat" (arguing) due to the entrance exam pressure. In the light novel her mother made her and Hachiman leave so she could sleep in on the weekend. She gave Komachi money to spend but not Hachiman.
Hachiman is Komachi's older brother. Hachiman seems to dote on Komachi a lot while Komachi cares for and relies on her brother as well. She understands that her brother has a corrupted personality and tries to get him past it.
Komachi often tries to get Hachiman to lose his rotten personality and be more sociable only to be ignored. Even though she cares for her brother a lot, she enjoys teasing and mocking him. In the light novel, it is mentioned she enjoys staying at home with her brother on weekends.
Hachiman highly cares for his sister which is often shown in the series. His acquaintances sometimes teasingly refer it to as sis-con though he denies as he has no interest in her like that. It was mentioned that during her elementary school days, Hachiman took care of the cooking while their parents were away at work. Although now these duties have been taken over by her.
In the past, due to the absence of their parents, Komachi would often come home to an empty house. She hated feeling unloved and being constantly alone. One day, she finally ran away from home. Hachiman was the one to find her, and bring her back home. Ever since then, Hachiman started coming back home from school early before Komachi so she doesn't have to come back to an empty house. However, its a lie told by Komachi so that Saki Kawasaki can understand her younger brother's concern for her, as Hachiman is surprised and has no recollection of the incident at all. He does however, remember coming home early to Komachi, but this was because he had no friends to hang out with.
She understands the pain Hachiman deals with and why he can be rude to others, but acknowledges that her brother has a kind heart. She often meddles in his life by using her sly tactics to change him as she tries to pair him up with girls like Yukino or Yui so he could be happy with someone but is frustrated with his lack of interest in the matters. Unknown to her, Hachiman is actually well aware of her true motives and is really annoyed by her attempts to set him up because of his past failures. It appears the only reason he hasn't stopped her is because he somewhat appreciates it as she's looking out for him.
In the OVA, Komachi mentioned their grandparents, she says they are sweet, but Hachiman explains that they tend to be sweeter with the younger one.
Komachi seems to have fine relationship with all acquaintances of her brother, especially the Service Club members. She has even exchanged contact ID's with all of the service club members. She helped the service club in preparing a magazine as part of a campaign by the Chiba region to promote "Love and Marriage" to teenagers.
Komachi went out with the Service Club for the first Shrine visit of the new year and also to Chiba summer camp activity. She sometimes slyly sets up date-like situations involving her brother and any girl of the Service Club during their outings.
Yukino once mentioned that she was jealous of the sibling relationship between Hachiman and Komachi, but they seems to be on good terms. Komachi has high respect for Yukino and just like Yui, she often tries to pair up her brother with her as shown when they went shopping or to the shrine.
They freely talk about various topics and it is shown that Yukino genuinely cares for Komachi, which is notable due to her usual distant nature towards others. They talk to each-other on a first name basis.
It was revealed that Yui and Komachi have met each-other before Summer Camp. After Hachiman's car accident, Yui came to thank Hachiman for saving her dog but found him "sleeping", she then gave the sweets as thanks to Komachi and left without Hachiman knowing she ever came to his house.
They met again and Komachi recognizes her at a tea shop before innocently informing her brother that it was Yui who was the girl that visited their house. They seem to be on good terms, as they call each other on a first name basis and were seen playing with each other in Chiba village during an outing.
Yui even left Sablé under the care of Komachi during her family's summer vacation. Komachi is more relaxed to hangout with her than Yukino due to Yui having a more open and friendly attitude. Komachi is well aware of Yui's feelings for her brother as she took note of Yui's nervous behavior at the suggestion of meeting their parents and wasted no time trying to set the two on an outing.
In the drama CD, Komachi gifted Yui a photo frame for her birthday.
Komachi and Shizuka made their acquaintance and were mentioned to have exchanged their contacts. Shizuka slyly brought Hachiman to Chiba summer village camp with the help of Komachi. Shizuka uses Komachi as a means to keep track of Hachiman when he isn't at school. Komachi also constantly informs Shizuka about Hachiman which is mentioned during Chiba village trip. Shizuka also wished that she had a sister like Komachi but swiftly flustered and asked others not to think deeply about it.
Komachi was mentioned to be more social than her brother. She was mentioned to be friendlier with everyone in both class and prep school.
Komachi and Taishi go to the same prep school. She appears to be good friends with him. She gave him advice about his family issue and slyly makes her brother help him. Both Komachi and Taishi vow to go to Sobu High together, much to Hachiman's displeasure. In the light novel it is hinted that Taishi may have feelings for Komachi which displeases Hachiman further.
Komachi met Saika while helping with Taishi's request. Like her brother, she too mistakes him to be a girl at first but was surprised to find out he was really a boy. She feels Saika is a nice person (mentioned in OVA). She invited Saika to be a judge for the Wife point contest and Komachi also played with him in river bank of Chiba village camp. Despite all of this, Komachi worries about her brother's close relationship with Saika. They are on good terms and address each other by their first names.
In light novel, Hayama mistakes Komachi to be Saika's sister, due to their closeness.
Komachi made her acquaintance through her brother, Taishi. They started with indifferent but later developed respect for each other. Saki views Komachi in good light and heads to her request in times of Iroha's request. Once they met at a restaurant during the time of summer break while attending cram school. Where Hachiman was angry at Taishi for calling him brother, Komachi innocently pointed out the possibility of Taishi calling Hachiman brother, if both Saki and Hachiman ends up together, causing Saki to be agitated, blush and fluster.
They first met at Chiba Village summer camp and their interaction are very little. Hayato mistook Komachi to be Saika's sister but later made a formal greeting. The misunderstanding was cleared. Komachi felt Hayato's presence to be a bad news for Hachiman, as Komachi felt that Hayato has more chance with girls than her own brother. This stopped after Hachiman stated to her that he isn't worried about Hayato or his popularity.
Not much is mentioned of their relationship but it was shown that Haruno and Komachi exchanged their contacts during the Sobu High's cultural festival. They have many common traits such as the sister of the two main characters, and slyness of great degree with a hidden motive.
They were shown to interact most in the Drama C.D of Volume 7, to which Hachiman refers to them as a "Dangerous Combo". Both seem to enjoy the romantic tension between Yukino, Yui and Hachiman. Komachi sees Haruno as another candidate for Hachiman.