Mio Akiyama
Ritsu and Mio Mio and Ritsu are childhood friends, but the latter could not remember how they first met.  In elementary school, Ritsu often picked on Mio due to Mio being very shy, as well as left-handed and having silky black hair, among other things. Their first 'serious' interaction came when Mio won a prefectural award for an essay she wrote and she had to read it aloud in front of the school. Mio was depressed because of her shyness, so Ritsu encouraged Mio to imagine the audience as pineapples, which allowed Mio to get over her stage fright. Since then, the two have been best friends despite their very different personalities. Ritsu has loved to tease and scare Mio since they were kids, especially with horror stories and gross things like barnacles. 

Ritsu also clings onto Mio like a lifeline when it comes to studies, and claims that Mio is the best tutor she has ever met. There have been times when Mio clings onto Ritsu instead; when Mio was being bullied by Sawako or other people, Mio would come running to Ritsu hoping she would save her from distress (although sometimes Ritsu joins in the bullying too). They both also have a similar interest in photography, as Ritsu always brings a digital camera with her, and Mio brings a film camera around. Ritsu was also the one who introduced music to Mio, and encouraged her to learn bass.

Ritsu's closeness, and sometimes codependency with Mio, is showcased when, after seeing Mio spending more time with others like Nodoka, her body reacts and she becomes ill in an attempt to win back Mio's attention.

Ritsu and Mio understand each other so well that they can easily imitate each other, and at the same time act as Romeo and Juliet for their school's festival performance. Ritsu is able to recognize Mio by the sound of her footsteps.

There were times when Ritsu got jealous when Mio was spending more time hanging out with others than with her, resulting in a brief conflict between them. However, such tension between the two does not last long.

During Mio's Fan Club Tea Party presentation, their other band members questioned if the slide show was really about Mio, since most of the pictures were dominated by Ritsu. Ritsu said that the photos were from her collection, which she had with Mio since they were little, therefore Mio appeared in every single photo. The fact that there were so many photos of them them together showed how they grew up together as best friends.

Yui Hirasawa
Yui and Ritsu

Ritsu's first impression of Yui was that she was a clumsy, childish, and useless girl. But when Yui joined the Light Music Club, Ritsu stated that she misjudged her. Yui and Ritsu have a lot in common, such as their lazy and laid-back attitudes, reluctance towards learning (or school in general) and their need for fun. They also have a sweet tooth for Tsumugi's treats and are usually the band members who propose a break first.

Yui sometimes gets disappointed when she finds out that Ritsu has also a reliable side, like when she does well on her exams or 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
Mugi-Ritsu Ritsu first met Tsumugi when Mugi originally intended to join the choir club. Mugi later joined Light Music Club only after witnessing the "fun and painful interaction" between the Ritsu and Mio. Ritsu had usually been polite to Mugi. The only exception was when Mugi suggested that if Ritsu was male, she would be really popular among girls. It was the one and only time Ritsu hit Mugi on the head and shouted at her.

Although the two do not have the closest relationship among the members of the Light Music Club, Mugi has always enjoyed Ritsu's company. When the two ran into each other one day and decided to go out together , Ritsu had brought Mugi to commoners' places like tuck shops and the arcade, as she knew that Mugi was a rich girl and rarely got to visit those places.

Ritsu had once said that even without Mio, she still has Mugi to tutor her in her studies, meaning she sees Mugi as a very reliable person (unlike herself).

Ritsu was shocked and speechless when she first found out in a phone conversation with Azusa that Mugi's house had a housekeeper. She is often amazed by Mugi's life standard which differs so much from her own.

Azusa Nakano
Ritsu-Azusa After Yui, Ritsu is can be considered Azusa's next closest senpai (probably because of Ritsu's friendly nature as well). Whenever Azusa laughs about Ritsu or nags about the Light music club, Ritsu pull her into a choke-hold as punishment (which happened more often in the manga than in the anime).

Azusa badmouthed once about Ritsu being a "lazy and irresponsible leader", although the latter does not truly seem to mind such comments. Azusa has praised Ritsu's cooking, yet, in the same episode, Azusa was the one who pointed out Ritsu's father's underwear in the living room.

Despite being often disappointed by the club president's irresponsibility, Azusa still held her respect and admiration over Ritsu and the band's coherent performances. When Azusa first mentioned that she is joining the Light Music Club, Ritsu had practically pounced over her. Ritsu and Yui had been scolded by Azusa numerous times when they were lazing around and procrastinating during their training sessions and camps.

At the end of the series, Ritsu could be seen worried over Azusa being the only remaining member of the Ligh music club (although that would make her the club's president). Ritsu would act responsibly as the leader whenever the club needs to do something for their junior. The most memorable scenes were when Ritsu went all out trying to recruit freshmen for Azusa and when she initiated a performance with the other band members when Azusa was considering quitting the Light Music Club.

Satoshi Tainaka
Ritsu with Satoshi Ritsu and Satoshi are close siblings living together. Ritsu, as the older one, often plans activities with her little brother like going to the cinema. Satoshi is often troubled by Ritsu's forgetful nature and has to help her out in such situations. Despite lamenting over it, he is still somehow willing to do so. The social life of his sister seems to interest Satoshi, as he is caught at least two times in front of her door when she had visitors (however, one time he blew his cover by laughing about his sister playing Juliet in her class' school act). Ritsu still can be angry with him (as seen when he laughed about her) and also has some authority over him as the older sister (as seen when he asked her if he could go visit a friend ). Satoshi is also an exceptionally reliable brother, going as far as to find and deliver Ritsu's student handbook to the passport registry.
Sawako Yamanaka
Ritsu blackmails sawako Ritsu often sees Ms. Yamanaka as a feckless teacher and is more than once out of words when Sawako asked herself about her personal life. Despite Sawako being a teacher and is therefore an authority, the two have easygoing relations. Ritsu was the first to call her "Sawa-chan" and is the member of HTT who is the most bold toward her (forcing her to become the supervisor of the Light Music Club, calling her "idiot" after she yelled at Azusa, dressing her down when she was in a hangover, or trying to swallow a receipt that she had to give to Sawako). Sawako however is able to gain complete authority over Ritsu when she gets angry, and she also scares her a lot when she appearing out of nowhere.
Nodoka Manabe
Ritsu and Nodoka Nodoka and Ritsu do not get along very well initially due to Ritsu's laziness and her attitude to often forget to hand in forms for festivals and concerts; and on top of that Ritsu also was envious of Nodoka since she saw Nodoka as a rival regarding her friendship with Mio. But since Nodoka was willing to shoulder Ritsu's fault (once again forgetting to hand out a form for the school concert) alongside her, Ritsu dropped her prejudices and the two have generally good relations.
Ui Hirasawa
Ritsu and Ui Ritsu and Ui do get along well. Ui respects Ritsu as a formidable upperclassmen despite her immature nature and is thankful to her for being such a good friend with her sister Yui. Ritsu is always amazed by Ui's housekeeping skills and told Yui more than once that she is envious and "also wants an Ui at home". At one time the two played video games together and Ui beat Ritsu in-game, much to the grief of the latter.
Ritsu and maki Ritsu and Maki are friends from middle school. They are still in contact despite attending different schools now, seen when Maki invites Ritsu and her band over to play at a Live House. When the two groups met coincidentally in London, Ritsu was elated to see a familiar face.
Ayame Yoshida
Ritsu and Ayame Ritsu and Ayame got along the moment they first met since both share a similar personality and are also very alike in other sectors like, for example, clothing. Ayame likes Ritsu's outspoken personality, especially towards Akira. The two also share another physical trait, namely their flat chests and constantly need to sheer up the other because of it, especially when Mio and Sachi are around.
Megumi Sokabe
Ritsu and Megumi Ritsu and Megumi became close friends even though the latter was already attending university. Since Ritsu wants to visit the same university, she asked Megumi to lend her important material, which Megumi glady did. The two hang out so often that Mio thought that Ritsu had a boyfriend.
Jun Suzuki
Ritsu and Jun Ritsu and Jun know each other through Azusa (however Ritsu tends to forget Jun's surname and calls her "Sasaki"). Jun's first thought about Ritsu were that she seems like a "very cool" person. Her point of view was corrected, however, by Nodoka who scolded Ritsu for forgetting to hand out a request form.