She seems to be good friends with Hayato Hayama, Yui Yuigahama, Hina Ebina, Kakeru Tobe, Ōoka, and Yamato. She often hangs out with them in and outside of school. She cares for and looks over her friends.
Yumiko seems to harbor feelings towards Hayama, as all her negativity in her personality doesn't seems to flow in the conversation with him. Most of the time, she blushes in their conversations and cannot refuse any of his requests. She also stands up for him whenever he is bad mouthed or insulted. This is shown by her hostility towards Yukino in the Chiba village camp and also towards Saki during the field trip grouping.
They seems to be closer than any other friends in the clique. Yumiko also shown to be upset with Hayama's double date with Orimoto, and also by the rumors of him dating Yukino. However, she was unable to ask him anything about it.Yumiko felt the disturbance of Hayama of the rumors of him and Yukino, Yumiko understand that he is in pain.
Yumiko thinks very highly of Hayama, this can be seen when she relaxed after hearing Hachiman's lie that Hayama was planing something to save their clique in the field trip.
Yumiko also tried to ask about Hayama's plans during Christmas, indicating her interest in spending her time with him at the time. Yumiko felt that Hayama distanced himself from the rest of the members and filed a request to the service club in order to find out Hayama's career path, so that she can choose the same with the possibility of continuing a year with him.
Her feelings for Hayama is known to all her group members. Hayama and Yumiko call each other on first name basis.In later volumes Yumiko is shown to bridge the gap between herself and Hayama with no result.When Hayama crushed the rumors about him and Yukino by using the cheers of Yumiko and Iroha, to which most of the school was convinced that the rumors were lies based on their observed closeness between Hayama and Yumiko
While making valentine chocolate for Hayama, she also showed shocked reaction after learning about Yukino giving Hayama some valentine chocolate in the past and later relaxed after Hayama's explanation.
She is one of Yumiko's close friends. Yui's attempt to eat lunch with Yukino causes some strain in their relationship, which is slightly continued up to the tennis court incident. But later on, the strain was cleared and their relationship deepens further, as Yui was able to speak of her mind more clearly, this results in Yumiko treating her more gently. In the Light Novel, they hangout together, during summer vacation and other outings.They are on good terms, and call each other on first name basis.
She also scare the ones who tried to dump the work load on Yui and also against sagami's clique's Sardonic laughing on Yui.
Once while Yui enquiering the girls for hints of chain message, where Yui almost revealed that she likes someone which made Yumiko pestered to know who it is with a promise to help her but Yui averted the subject and the topic was never brought again. There are hints of Yumiko knowing about Yui's feelings for Hachiman but decided to stay ignorant of it, just like her role in Tobe's feelings for Ebina. In the field trip, Yumiko asked Hachiman if he didn't learn anything from his relation with Yui in an attempt to explain him the situation involving Hina but later brushed it off.
Yumiko describes her as the quiet opposite of Yui, who blends in without reading the atmosphere. Yumiko doesn't seems to mind her yaoi interest. Frequently, Yumiko is the one to stop Ebina from going crazy with her fantasies. In the past, many boys approached Yumiko in hopes of being introduced to Ebina, which caused a slight strain in their relationship. Later, Yumiko avoided such situations in a way of not losing her.
Yumiko is few of the people to know the crafty, sly and mindset of Ebina. Yumiko cares for Ebina greatly. Ebina shows her true care towards Yumiko as well.They call each other by first name basis. They are also shown to be shopping together with much comfort.
From the beginning of their encounter, they show great enmity towards each other because of their same social status, contrasting personalities and different views of life which results in a rivalry. This enmity grew after each encounter. Like in the tennis court showdown, Chiba village summer camp, field trip, etc. After hearing the rumor about Hayama and Yukino dating, Yumiko was enraged to the point where she grabbed Yukino by her collar after a short verbal spar. However, this was blocked by Yukino. Yumiko suspects that Yukino and Hayato have some sort of history similar to the rumors without knowing any of their past acquintance based on Hayato's pained reaction on the rumors.But Yukino answered coldly in a round about manner enraging Yumiko.
After their misunderstandings were cleared, they grew softer towards each other. Their hostility gradually dissipated, as Yumiko apologized for her past actions, and personally invited Yukino to the party, causing Yukino to smile and Yumiko to blush, which surprised Yui.
In episode 12 of season 2, Yukino helped Yumiko to make valentine chocolate where their interaction seems to be normal.
Yumiko and Iroha compete for Hayato's attention, which is clearly visible in their trip to the Destiny Land. In spite of this, Yumiko shows genuine motherly care for Iroha when she was feeling dizzy due to her rides, surprising Iroha herself. In the Light novel, Yumiko goes in search of Iroha after Hayama's rejection of Iroha to confort her. Hachiman also felt that Yumiko is the better choice for the job.
In season 2 episode 12, Iroha and Yumiko both make cookies and both have Hayato test taste but they are seen giving glares at each other, implying that their tension for each other is still there. both of them acknowledged each other as love rival.
According to Hachiman's words, he and Yumiko neither are on good terms nor bad terms but definitely not in the neutral state as well. Yumiko Is mostly indifferent towards anyone she considers inferior in social status and doesn't view Hachiman in good light, due to his creepy looks and personality. She grows to despise him further as he grows closer to Yui. Hachiman himself seems to reciprocate this dislike but merely ignores her. Yumiko doesn't concern herself with others, who are not in equal social standing and thus ignores Hachiman's existence.
After the incident that lead to the strain between Yui and Yumiko, she glared at Hachiman at the tennis court showdown for a second and also to Yui. She also openly humiliated Hachiman in-front of the spectators that no girl who would pair up with Hachiman for the mixed tennis match, to which Hachiman himself laughed along with spectators, to her annoyance. Later, she was taken back by Hachiman's last two serves. During the summer camp, she glared at Hachiman for letting Hayato do all the work, even though Hachiman did most of the work before her arrival. Later, both of them completely ignored each other as they had no reason to involve with each other.
In Kyoto school visit, she explained to Hachiman, the consequence of Tobe's confession to Hina and asked him not to meddle with her clique in an angry tone. When Hachiman promised Yumiko that her clique will be unaffected, Yumiko questions as if it is given. Realizing that Yumiko doesn't have any faith in him, Hachiman lies that Hayato planned something, causing her to smile in relief.
During their Destinyland visit, Yumiko showed her unpleasantness for having Hachiman there with them, even though she didn't seem bothered having Iroha (love rival) or Yukino. Hachiman gave a troubled smile in response but was really not thrilled to have entire Hayato clique with them as he only anticipated Hayato alone by iroha's handy-work.
She finally relied upon Hachiman through service club to find Hayama's career path. Later, Yumiko tried her hardest to avoid any eye contact with Hachiman after he saw her teary eyed, out of pride. After seeing her maiden side, Hachiman for the first and only time though Yumiko to be cute. After finishing her request, she gave a half-hearted thanks to Hachiman alone but a hearty one to other members. As day by day, Hachiman notices that Yumiko views him without any scorn unlike the previous time when he was with Yui