Both of Komachi's parents are unnamed and are working in a corporate company, so they are always either late or outside, thus never shown in the series, to which Hachiman refers to as corporate slaves. 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.
Her father shows to much kindness towards Komachi and was much pitiful when she refused to talk to him.Her father even threatens any boys that comes close to Komachi even Hachiman her own brother.
Her mother was mentioned in episode 7 of season 2 where she asked Hachiman through Komachi to buy chicken and cake for christmas. In episode 13 of season 2 Hachiman mentioned that Komachi and their mother are at each others throat due to the entrance exam pressure.
In the past, due to the absence of their parents, Komachi often comes home to an empty house. She hated feeling unloved and being constantly alone; one day, she finally runs 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, it is a lie told by Komachi so that Saki Kawasaki understand her younger brother's concern for her, as Hachiman is surprised and has no recollection of the incident at all.
She understood the cause and pain of Hachiman resulting him in current state and wishes to change him. She also acknowledges her brother and his kind heart. She uses her sly tactics to change him.
Hachiman also highly cares for her sister which is often shown in the series, to which his acquaintance teasingly refer it to as sis-con. It was mentioned that during her elementary school days, Hachiman took care of the cooking, while their parents were away in work considering her needs and safety, which is now taken over by her. Additionally Kawasaki Saki of class 2F developed respect for Hachiman, after meeting her. Komachi is the only person in the series to wish Hachiman for his birthday by text.
They even went out together for Shrine visit for new year and also to Chiba summer camp activity. She sometimes slyly setup date-like situation with her brother and any individual of the Service Club's members during their outing.
Yukino once describes that she is close to her who takes care of one loser (Hachiman) by which she knows the Hardships of married life.
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. Both of them call each other on a first name basis. In the drama CD, Komachi gifted a photo frame for Yui's birthday.
Komachi was mentioned to be social than her brother but she too just like Haruno didn't establish an equal relationship with anyone.She was mentioned to have friendlier chat with every one in both class and prep school.
OVA. She invited Saika to be judge for Wife point and Komachi also played with him in river bank of Chiba village camp. Despite all of this, Komachi worries about her brothers's romantic interest towards 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 and Shizuka made their acquaintance and each have mentioned to have exchanged their contacts. Shizuka slyly brought Hachiman to Chiba summer village camp with the help of Komachi. Komachi also constantly informs Shizuka about Hachiman which is mentioned during chiba village trip.In light novel Hachiman's advice to Taishi about choice of high schools was also informed to Shizuka by Komachi.
Like all characters Shizuka is also jealous of this siblings relation and praises Komachi and expressed her wish for a sister like her
They first met at Chiba Village summer camp and their interaction are very little. Hayato mistaken Komachi to be Saika's sister but later made a formal greeting, once the misunderstandings were cleared to which she hid behind Hachiman while giving her reply. When Komachi gave her opinion on Rumi's predicament, Hayato agreed with her.
It was not mentioned of their relationship but it was shown that they exchanged their contacts during the Sobu High's cultural festival.Both of them have many common trait such as the sister of the two main characters and exhibition of slyness of great degree with much hidden motive. They were shown to interact most in the Drama C.D of Volume 7, to which Hachiman refer to them as "Dangerous Combo". Both seems to enjoy the romantic tension between Yukino, Yui and Hachiman. Komachi sees Haruno as another candidate for Hachiman.