Voiced by: Minako Kotobuki (Japanese), Shelby Lindley (English)
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. 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. Mugi has long, wavy blonde hair and sky-blue eyes. 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.
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 risqué in her head (both Mio and Ritsu find this awkward at times). 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.