Tsumugi "Mugi" Kotobuki is one of the main characters from the K-On! anime and manga.
The soft-spoken keyboardist who frequently provides tea and sweets in the middle of band practice. Hailing from a very wealthy family with a large business empire, she takes great delight in the more mundane aspects of middle-class life, being so separated from it normally. Extremely kind, she's always willing to use the resources she's been blessed with to help the club.
In the AWA fics, Tsumugi is a student at the AWA Academy.
In the XP4 fics, Tsumugi is a student at the Vanguard Academy.
Mugi has long, wavy blonde hair, sapphire-blue eyes, and a very fair complexion absent from the other girls of HTT. She also has thick yellow-brown eyebrows that remind Yui of pickled radishes. Skirts over pants. High society lady styles, such as elegant one piece dresses.
She wears her Sakuragaoka uniform consists of a navy blue blazer, a solid white buttoned shirt (under the blazer), a colored ribbon (different colors for different student years, Mugi's year is light blue), a light steel blue skirt, stockings or socks (Mugi's are white), and maroon shoes.
Tsumugi, who often goes by "Mugi", is the keyboardist of Ho-kago Tea Time. Mugi is a wealthy girl who has a sweet and gentle personality. She has been considered a piano prodigy since she was four, and has won many piano recital contests. As the daughter of a company president, Mugi and her family own several villas in various places around Japan (and even one in Finland, much to Ritsu, Azusa and Ui's surprise when they tried to contact her at home). She often brings sweets and confectioneries to her friends, which usually ends up distracting them from practicing. 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 entranced by the sight of two girls interacting closely together, sometimes imagining something more risque. She originally intended to join the choir club, but after Ritsu's false flashback of promising to start a band and seeing Ritsu and Mio's silly interaction, she decided to join the Light Music Club instead. 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 Mio to hit her), since she has little opportunity to experience them with her high status.
Despite being wealthy, she tried 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.
Another characteristic that distinguishes Mugi from most girls her age is her surprisingly prodigious physical strength. Her astounding feats of strength include rapidly carrying various pieces of Ritsu's drum set, one by one, with seemingly little effort and even humming a happy tune throughout; abruptly defeating the arm wrestling machine on maximum setting at the Game Centre, which Ritsu had painfully struggled against; and sending Akira flying into a wall with a "light" slap. Furthermore, she regularly runs a brisk jog with her instrument, the Korg Triton Synthesizer, slung across her shoulder. (The Korg Triton is a hefty piece of equipment: at seventeen kilograms, its weight is comparable to that of a FIM-92 Stinger Missile or FGM-148 Javelin Missile Launcher. Additionally, she did not tire or slow down while dragging thirty kilograms of luggage and carrying her Triton Synthesiser over her shoulder, as the band was running to catch a Taxi to London Heathrow Airport.
- Her keyboard is a 76-Key version of the Korg Triton Extreme, which is now discontinued. It is a very heavy instrument, as noted below.
- Her surname is taken from former P-Model keyboardist Hikaru Kotobuki. Mugi's last name also has the same pronuciation as her voice actor Minako Kotobuki, but is written with different kanji.
- The kanji of Tsumugi's given name refers to pongee - soft, thin cloth woven from raw silk.
- 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 and informs her that Mugi is on an overseas trip in Finland. 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. However, upon entering university, Mugi decided that she wouldn't depend on her wealth anymore and told her parents that the only thing she needed them to pay for would be her tuition.
- At one point, Akira bluntly points out that the girls of HTT might be friends with Mugi because they're after her money, making Mugi cry. Yui and Mio later reassure her that they're friends regardless of money, and then immediately afterwards show excitement over some sweets she brought for them. Mio remarks that they're not after her money, but her sweets.
- Mugi sometimes has to use the sweets and beverages she brings to manipulate the others (especially Yui) into doing club activities they would normally be lazy to do.
- Mugi is apparently very strong, as in the anime she is seen carrying loads of heavy equipment without breaking a sweat, including her Triton Extreme, which weighs 16.9kg / 37.26 pounds. She also knocked Akira over with a "light strike".
- In the AWA fics, Tsumugi became Ringo from Flash Drive's girlfriend.
- In the AWA fics, Tsumugi is friends with Kaguya Shinomiya and Chika Fujiwara, due to their families being connected to business.