Tsumugi, more known as "Mugi" is the gentle and sweet keyboardist of the club. Mugi is a wealthy girl who has a sweet and gentle personality. Mugi is considered a piano prodigy since she was four and has experience winning in piano contests. She is the daughter of a company president, and her family has several villas in various places around Japan (and even one in Finland, much to Ritsu, Azusa and Ui's surprise when they try to contact her at home). She often brings sweets and confectioneries to her friends.The tea set and the tea which they drink often are unusually high in price. Although she is a sweet and gentle girl, she is often weirdly entranced by the sight of two girls interacting closely together, sometimes imagining something more risque in her head (both Mioand Ritsu find this awkward at times). 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. Mugi displays a rebellious streak occasionally, due to her wanting to deviate from her wealthy nature, and diverts from a well-behaved and mature demeanor to the surprise of others. She also has a habit of getting a little too enthusiastic about experiencing new things that the others take for granted (such as ordering fast food, eating yakisoba at a festival, and purposely provoking Ritsu to hit her).

Despite her being wealthy, she also tries her hand at a part-time job at a fast-food restaurant.^[1] 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.