Tsumugi, often referred to as 'Mugi' by her friends, is a wealthy girl with a gentle and sweet personality who plays a Korg Triton Extreme 76-key keyboard, though she is also seen playing a Korg RK-100 keytar in the closing credits of the first season, and a Hammond organ in the closing credits of the second season. She originally intended to join the choir club, but joins the light music club instead after receiving an invitation and encouragement from both Mio and Ritsu. Tsumugi is considered a piano prodigy since she has been playing the piano since she was four and has experience in winning various piano contests. Tsumugi has written several songs for the light music club, as well as singing the back vocals in several other songs. She is a good student and has long, pale blonde hair, blue eyes, unusually large eyebrows, which apparently run in her family and a fair complexion that the other characters do not have, but she does not get sunburned.
She is the daughter of a company president, and her family has several villas in various places around Japan (and even one in Finland). Since her father also owns a maid café, she often brings confectionery and an assortment of sweets and pastries to the club room, and she diligently makes tea with a tea set which is kept in their club room. Despite her wealth, she is fascinated by and finds joy in "normal" activities, such as ordering fast food, sharing french fries with her club mates, holding down part-time jobs and haggling over prices. Tsumugi displays a rebellious streak occasionally, diverting from her normally well-behaved and mature demeanor to the surprise of the others. She also displays a childlike eagerness from time to time, and possesses unusually high strength, being able to effortlessly carry around her own keyboard, Ritsu's drums, amplifiers, and at one point beating an arm-wrestling game in an arcade.
Tsumugi is often entranced by the sight of two girls interacting closely together, sometimes imagining something more risqué in her head. The series sometimes indicates that Mugi has a crush on their teacher, Sawako Yamanaka. While a lot of things do not bother her, she is fairly conscious about her weight (just like Mio), and she gets a bit anxious when her family's staff start spoiling her friends during villa visits. She later begins learning how to play the guitar from Azusa. Tsumugi has a childhood friend several years younger than her named Sumire Saitō, who is a daughter of the family that serves the Kotobuki household. Tsumugi grew up home schooled, and therefore spent very little time in the outside world. Because of their close friendship, Sumire would purchase everyday items for Tsumugi, like manga that she was not normally allowed to see. Tsumugi was rather taken by some of the yuri manga she received, which may have influenced her later perceptions about relationships. When Sumire started high school, Tsumugi wanted her to experience the light music club on her own, but due to Sumire's shyness Tsumugi came up with the excuse of sending her to the club room to pick up the tea sets that had been left there and told Sawako when Sumire did join that it was okay to leave the tea sets there.