Mugi first met Yui back in the Light Music Club room and spoiled her with sweets and tea so that she would be interested in joining the club. After Yui joined, Mugi did quite a lot of kind things for Yui, such as lowering the cost of the guitar Yui wanted to buy.
Yui always wanted to taste more of Mugi's sweets, and still can't wait to eat them. It is said that, without Mugi's snacks, Yui wouldn't be able to play the guitar properly.
Like Mio, Mugi is concerned about her weight and can sometimes get very annoyed when she sees Yui's fast metabolism.
Mio first met Mugi when Mugi originally intended to join the choir club. Mugi later joined the Light Music Club only after witnessing the "fun and painful interaction" between Mio and Ritsu. Mio and Mugi have been good friends, but sometimes Mio finds that Mugi "betrays" her. For example, when taking vote of either to play at the beach or to practice, Mugi decided to play along with both Ritsu and Yui, and Mio felt Mugi betrayed her (in kind form).
Like Mio, Mugi is very concerned about her weight, and both are jealous about Yui's fast metabolism.
Ritsu first met Mugi when Mugi originally intended to join the choir club. Mugi later joins Light Music Club only after witnessing the "fun and painful interaction" between Ritsu and Mio.
Ritsu was shocked speechless when she first found out in a phone conversation with Azusa that Mugi's house had a housekeeper.
Ritsu has always been polite to Mugi. The only exception was when Mugi suggested that if Ritsu was a male, she would be really popular among girls. It was the one and only time Ritsu shouted at and hit Mugi on the head, but no lasting harm was done. (Given that Mugi specifically requested to be hit in the episode/chapter, Mugi might provoked Ritsu on purpose)
Mugi treasured her friendship with Ritsu very much as seen that she always enjoyed Ritsu's company. Being as observant and wise, on one occasion, Ritsu brought Mugi to commoners' places like tuck shops and an arcade, as she knows that Mugi, being a rich girl, might not have visited those places before. Ritsu had once said that even if Mio is not available, she still has Mugi to tutor her in her studies.
Azusa finds Mugi to be very beautiful, and envies her hair and large eyes, and at one time started to teach her how to play guitar, when the two are alone in the club room. She also finds Mugi's snacks to be delicious and has enjoyed eating her snacks.
She was shocked to find out that Mugi's house had a housekeeper and was impressed with the villa that Mugi borrowed for the girls during the band's second training camp.
In the manga, before the final exams, Mugi and Azusa are often the only two to show up to the club room, with the others studying at home. In this time, they strengthen their friendship, with Azusa teaching Mugi to play guitar.