Ui Hirasawa
Ui hugging Yui Ui is Yui's younger sister. But instead Ui acts more like the "older sister" of Yui, since she is more responsible and mature and takes care of her older sister. Yui is quite happy to be able to have Ui as a sister and loves her very much. This is proven when she decides to make a meal for Ui when she got sick taking care of her despite her lack of cooking skills compared to Ui. Yui even dedicates one of the very first songs that she writes to Ui.
Nodoka Manabe
Nodoka is in class 3-2 too

Yui and Nodoka are very close friends, having known each other during kindergarten and have been childhood friends since then.

The closeness of their friendship can be evidenced by the fact that they went to the same Junior High and even the same High School together. The two are mostly seen together when Yui isn't with the rest of the Light Music Club members.

Mio Akiyama
Mio and Yui singing 2

Yui and Mio are often known for not being on the same side. Mio's maturity often doesn't work with Yui's childish personality. Yui tends to only want to play and keeps forgetting to practice, which usually makes Mio annoyed.

During their first meeting, Mio taught Yui the basics of playing guitar and the basic guitar chords, which Yui thought it were too tough for a beginner like her.

Mio even often got jealous of Yui for having a fast metabolism. Mio is very concerned about her weight and tries  not to eat anything that will increase her weight. On the other hand, Yui eats everything, but never has any increase in weight, making Mio annoyed and jealous.

Despite this, Yui has managed to impress Mio with her ability to learn things quickly. And during their band's first concert, when Mio was supposed to sing but was suffering from stage fright, Yui told Mio that she believed in her and knew that Mio had been practicing the song. The other band members followed suit, and the performance was a success.

Ritsu Tainaka

Ritsu's first impression of Yui was that she was a clumsy, childish, and useless girl without tempo. But when Yui joined the Light Music Club, Ritsu stated that she misjudged her. Soon, they became really good friends. Yui and Ritsu have a lot in common, such as their lazy and laid-back attitudes. They also have a sweet tooth for Tsumugi's treats, and are frequently the first to propose a break during practice.

Yui usually thinks that Ritsu is in the same boat as her and got disappointed when she found out that Ritsu does well on her exams and takes good care of her drums.

Despite acting immature herself most of the time, Ritsu sometimes criticizes Yui's odd behavior or thoughts.

Tsumugi Kotobuki
Yui and Tsumugi Tsumugi first met Yui back in the light music club room and had spoiled Yui with sweets and drinks so that Yui would be interested to join the club. After joining, Mugi had done quite a lot of good work for Yui, such as lowering the cost of the guitar that Yui wanted to buy.

Yui has always liked Mugi's cakes and cannot not wait to eat them. It is said that, without Mugi's snacks, she would not be able to play the guitar properly.

Like Mio, Mugi is very concerned about her weight, and will get very annoyed when Yui talks about her fast metabolism.

Azusa Nakano
Yui apologises to Azusa

After watching the Light Music Club's performance during the freshman reception, Azusa thought of Yui as a very talented guitarist, eventually this thought led her to join the club. Soon, however, she regretted thinking highly of Yui and now thinks of her as a student who just likes to procrastinate.

Azusa admires Yui due to her certain abilities, such as her ability to go far due to sheer determination, her perfect pitch, and her affection towards her friends. Despite being better at guitar than Yui, she insists that Yui played the lead guitar due to her seniority (though in some songs, usually opening songs, Azusa plays the lead guitar instead). Before Azusa's first concert, Yui was sick and unlikely to be able to play. Azusa was the one who brought up the idea to cancel the concert, since it would not be good for anything if the whole band would not be present in her eyes. When Yui actually showed up, Mio told her to apologize to Azusa since she was the one who worried the most about her.

Yui had become attached to Azusa immediately, often hugging her, and gave Azusa the nickname "Azu-nyan" (-nyan is a Japanese suffix associated with cats) after convincing her to try on a pair of cat ears and say "nyan", which would continue to be used by Yui throughout the series. Azusa would initially be annoyed by Yui's clingy behavior but grow accustomed to it and her nickname over tiime. However, when Yui crosses the line, (by trying to kiss her for example), Azusa will not hold back using violence to stop her, like slapping her face before the school concert or elbowing her in the stomach (when she mistakenly thought Yui was about to kiss her instead of her guitar).

Yui's laid-back personality does not usually work well with Azusa's more diligent character, but they have become quite close and managed to cooperate at times, like when they performed in a local talent show for Yui's neighbor. Overall, despite trying to hide it through her strictness, Azusa is fond of Yui and they become very close, even rooming together during the band's trip to London .