Character :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. Mugi has a long, wavy blonde hair and sapphire-blue eyes, and she has a very fair complexion that the other girls don't have. 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. 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 Mio and 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. 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...
- Her surname is taken from former P-Model keyboardist Hikaru Kotobuki. Mugi's last name also has the same sound as her voice actor Minako Kotobuki, but different characters.
- Although Mugi does not like to flaunt the fact that her family is wealthy, it becomes rather obvious at certain points, and always to the shock of her friends. Her butler answers the phone when Ritsu calls her at home once. Mugi's father owns the music shop that she and her friends frequently visit, so the workers there are always very courteous to her, and greet her using her proper title of "Lady Tsumugi". As a result, they offer her and her friends large discounts or free service.
- Mugi sometimes has to use the sweets and beverages she brings to manipulate the others (especially Yui and Sawako) into doing club activities they would normally protest to.
- Mugi can speak in Kansai dialect perfectly. This may be decided after Minako Kotobuki was chosen to voice her, since she too can speak Kansai dialect.
- Yui states that Mugi's eyebrows resemble pickled radishes due to their thickness. She furthermore emphasizes it in the K-ON! specials and dream sequences.
- Mugi's eyebrows serve as an anchor that prevent her from dissolving. When Yui ate her eye brows (assumed to be radishes) in one occasion, she started to melt.