Yui is the guitarist in the Light music club in the anime K-ON!
Yui is the main character in the K-ON! anime and manga. In her past she had never been in a club before and took life easy. When she entered high school she decided to join a club but due to her history of being laid back decided to find an easy club which she wouldn't need to put any effort into doing the activities.
Yui has a little sister who is a year younger than her named Ui, they look identical when Ui has her hair down. Ui has the complete opposite of Ui's personality though. She is kind and easy going, very mature, and she acts like a mother, doing all the housework and cooking as well as looking after Yui. She also has just as much if not more musical talent Yui has, being able to play the guitar very well when she pretends to be Yui in front of the other girls (and she was able to do this just by watching Yui practice).
Yui's mother and father are never seen in the series. They have been said to always be away on holiday's and work away.
Yui is a kind and friendly girl who is very lazy. She had been less lazy recently post joining the light music club than she had been all her life. Yui is not very smart and doesn't get good grades but when she gets help studying she gets a 100% for a test. She is very clumsy and spaces out a lot, often thinking about random and pointless things such as, what Mio's forehead looks like, what each of the girls in the group would be like if they switched personalities, what Mugi's eyebrows look like when they move, etc. That or she will just think about food.
Yui is easy going but can concentrate and work had very well when needed, though this normally leads her to forget anything else. She also has the rare ability of not gaining weight not matter how much she eats, this annoys many of the other girls.
She is very talented with music, picking up the guitar in a very short time, also being able to tune her guitar just by listening to it, something that Azusa was amazed with.
Yui decided to join the light music club thinking that she would be cable to play a castanet. She goes to the club and meets the members, where she has tea and treats which persuades her to join the club even though she can't play an instrument, she tries to quit soon after but feels bad that if she left the club would be disbanded, so she finally joined and started to learn the guitar.
Soon after the group go to buy Yui a guitar, she manages to get a raise of her pocket money to get the guitar but when they arrive at the shop theirs only one guitar that Yui is interested in and it is too expensive. The group then get part time jobs in order to raise money for it but after they find they still don't have enough money, the next day they go to buy another guitar but Yui is entranced by the same guitar again, Mugi then goes to bargain with the man working at the till who realises that Mugi is the daughter of the owner of the shop, he gives them a large discount on the guitar so Yui can afford it.
Soon after everyone has to take their mid terms, as Yui had been practicing guitar chords instead of revising she fails the exam, she is then forced to do a retake, if she failed the retake she would be banned from club activities which would cause the club to disband. She attempts to revise but each time she is distracted by her guitar, which she ends up playing, on the day before the exam, all the girls go to Yui's house and help her study.
Yui ends up acing the test with 100% but fails to remember anything else she learnt before including all her guitar chords. A while after Mio finds a box in the music room which has items belonging to the previous light music club, including a photo album and a cassette with some of their music on. Mio is inspired to play at the school festival and to be better than them so arranges a training camp. They all end up staying at one of Mugi's villa's near the beach where Yui spends most of the day playing on the beach. They all end up practising at the end of the day where Yui plays a perfect guitar solo and learns a new trick of vibrato.
When they all return they find out that they are not recognised as a proper club as they don't have an advisor, they go to ask Miss Sawako who is the music teacher but she refuses due to already being the advisor of the woodwind club. Yui and the others discover that she is in fact the old Heavy metal vocalist of the previous light music club, they use this to blackmail her into becoming their advisor due to her not wanting this info to ruin her pure image. Mio ends up writing lyrics for their first song but refuses to sing due to her shyness. Yui volunteers to sing but finds it hard to do the two things at the same time, singing and playing the guitar. Sawako gives Yui a training session to teach her how to do both and to make her better which seems effective, the only problem is due to the intense training Yui had lost her voice. Due to encouragement from her friends Mio sings well and the concert goes well.
Ui, Yui's sister remembers when they were little, she had wished for a white Christmas and Yui took the stuffing of the cushions and put them on the trees outside to try create a white Christmas for Ui, The girls decide to have a Christmas party together and end up having it at Yui's house, Yui wants to help with the cooking but due to her clumsiness Ui makes her do the decorating instead. in the gift exchange Yui receives a pair or mittens where Ui receives a scarf, the items they had lost before. They end up sleeping together, spending Christmas together, the next day they wake up to find a white Christmas.
Yui owns and plays a Heritage Cherry Sunburst Gibson Les Paul Standard electric guitar, she has named the guitar "Giita"
The Light Music Club members are approaching their graduation from high school, so they decide to go on a band trip to the United Kingdom.