In Drama C.D, Yukino mentions that it snowed at the time of her birth, so her parents named her Yukino which literally means snow in Japanese. Her family is on good terms with Hayato's family. Their families' relationship is not known by many in the school. In the past, Haruno would watch over both Hayama and Yukino when both of their families were in business. It was also mentioned that the trio has been to the amusement park where Haruno would mess with them, resulting in the fear of rides for Yukino.
Yukino studied with Hayama in elementary school. It was mentioned that in elementary school her indoor shoes were stolen from her about sixty times, but fifty of those times were done by girls in her class. Her upperclassmen harassed her frequently—all of this due to the fact that she is beautiful and famous, attracting unwanted attention from people.
In middle school, Yukino studied abroad (not mentioned where, when, or how long), and returned to Japan during her third year of middle school. She had to transfer into a random school, but all the girls in her class—or rather, all the girls of her school—were desperate to drive her out. None of them tried to prove themselves better than her, to whom Yukino refers as imbeciles. Generally, boys who approached her did so because of their feelings for her, which began from her final year of elementary school.
In the light novel, Yukino mentions that there were fellow peers whom she thought to be friends asked her for love advice. Yukino describes this as pretext of girls setting their boundary on a boy, so that other girls would be conscious of it and anyone who croses the boundary would be excluded from the circle. Yukino received the same treatment of a thieving cat even though she was on the receiving end of the confession leading to most gossip In Volume 7, Yukino describes her experience of being confessed by a boy in a girls point of view. Yukino states it to be a like a public humiliation and bothersome. She also adds on that it can be known in advance of who is going to confess, based on how noisy people are around you. The teasing and laughing are all things that you can hear.
It is hinted that Yukino once experienced chain messages harassment, due to her fierce hatred towards those who perpetrated it. She even believes that those kind of people need to be eliminated, which she claimed she did to the two perpetrators never fully introduced to the audience, Sagawa-san and Shimoda-san.
Yukino was constantly bullied for her excellence which is mentioned throughout the series and also pointed out by Haruno at firework festival. Once, Yukino compared the facial reaction of Yumiko in the tennis match to be similar to the ones who bullied her in the past. Hayama who was her classmate all the way from elementary school to the present was unable to solve her bullying problem. Hayama believes that if Hachiman was in the same school as them, things would have been different. In middle school, false rumors about Hayama and Yukino being a couple spread throughout the school.
According to Haruno, Yukino is mentioned to be relying on others more frequently in the past which is mentioned in Episode 11 of the second season. In the light light novel, Haruno mentions that Yukino used to be a cry-baby in the past, but Yukino denies it. Controversly in Episode 11 of season 1, when Minami went missing in times of closing ceremony of the Soubu High Cultural Festival, Haruno offered to help Yukino to buy time stating that, "it was the first time Yukino asking for help," but Yukino stated that she would just be indebted to Haruno and not a favour surprising her.
It was mentioned by Haruno that in Episode 12 of season 2 that Yukino gave chocolate to Hayama in Valentine's Day in the past. But Hayama adds on that Yukino gave it to both Himself and Haruno. Yukino also able to explain why Hayama is relucuant to take any valentine cookies that in junior high, as it left an strained atmosphere after the valentine, indicating some unknown past.
High School Life
It was Yukino's car that had hit Hachiman, from which he saved Yui's dog, on the first day of school. It was mentioned by Haruno that Yukino was in the back seat at the time of accident, but Yukino never speak of the incident to any others.
It was mentioned that Yukino is as popular as Hayama and Yumiko in Sobu High School.
Whatever the reason which is not shown to the audience, Yukino lives alone in an apartment bought by her father against her mother's disapproval
She started preparing tea, cookies, cakes and purchase snacks for her club mates and distribute to them as well. Yukino chooses International Liberal art course as her future career path which indicates that she gives up in following her older sister's shadow and follow Hachiman's future career. It was mentioned that Hachiman and Yukino can be together after all because they have the same career plan.
Haruno schemed to bring Yukino to their family's new year feast by holding Hachiman and Yui, who were out buying presents for Yukino's birthday. Yukino was hesitant to go with her mother but did so. This incident is witnessed by some one and resulted in a rumor of Yukino dating Hayato, who was with them along with Haruno to be in a family feast.
Later, in Episodes 11 of season 2 (adaptation of Volume 11), she was trying to give Hachiman her handmade cookies but failed due to her nervouseness and also because of her awareness of Yui's feelings towards Hachiman.
She chose Liberal arts as his third year course.