She first appeared in Episode 049 when Ikuto's violin breaks and he has no spare strings, he and Amu (who stayed around to sing along and listen to him play) go to Kotone's father's violin shop to have it fixed.
Kotone is shown to have green eyes and brown hair that's pulled into two ponytails with orange rubber bands to hold them near her shoulders; she also has a bang split in the middle of her hair so that they fall on either side of her face. When she is first introduced, she is wearing a pink shirt under long white shirt that stops below her stomach, but above knees. The white shirt has a V-neck and around it are some lowercase Xs on an orange background. At the end of her shirt is the same pattern of orange and Xs, but in a line. Her pants seem to be a type of purple color. Her shoes are light pink at the sides and white at the front.
When Amu comes back the next day to see how Kotone was doing, Kotone is now wearing a purple shirt with a white collar. On the ends of the purple sleeves are dark orange lines that start on the edge and go up a little ways before coming back down and over again. The color under the dark orange lines going up and down is a lighter orange. You could say they looked orange triangles connected together in a line. The bottom of her shirt has this same pattern and color. Her capris look white, but do seem to have a very light purple color to them. Her shoes are just like the other pair she had on, but they're orange at the sides instead of pink and have some yellow on them.
Kotone's X-Character looks like a typical one, black with a red "X" on their foreheads, but hers comes with a violin, much like her true appearance.
Kotone's Guardian Character is shown with a blue dress and blue shoes to match. She has purple eyes and a pink music note (an eighth note) above her left eye in her hair (most likely used as a hair clip) which is also pink and pulled into a bun with a long bang on the right side of her face. She also carries a white violin and does not mention her name to Amu.
Kotone works at her father's violin shop (serving tea to customers while they wait) where Ikuto used to be somewhat of a frequent shopper. Because of this, Kotone developed a crush on him and, when he came to get his violin fixed and brought Amu along, her father told her Amu was Ikuto's girlfriend, so she acted very rude toward her at first, having become very jealous. When she finds out Amu wasn't his girlfriend, she was embarrassed and began to act nicer to her.
After Kotone's father fixes Ikuto's violin, Amu is surprised at how fast he did it. Kotone passionately tells her about violins, saying she wanted to be a craftsman. She also tells Amu that since her father hadn't taught her very much about it, she had been studying in secret. A famous violinst who has been coming to this shop for a long time named Saeko Honda comes in and Kotone happily greets her. Saeko gives Kotone a violin in a blue case that she wanted adjusted, saying she needed it before she left for Paris. She then surprised Kotone by giving her a violin in a black case, asking Kotone to do it herself.
Saeko leaves and Kotone is still standing there, but when Amu runs over to congratulate her, she snaps out of it and looks excited. Ikuto tells the two girls that he was leaving and he tells Kotone to give his best to her father, but they were too busy talking about Kotone's first job request. Yoru points out that they were ignoring him and Ikuto, but the latter tells the former to leave them be and exits the shop with a smile. What Kotone didn't know was that her father had heard what Saeko wanted and was looking at her from his office with a smile before going back in and closing the door.
Amu asks Kotone if she could come back and look at the violin the next day, but Kotone was too busy focusing on the violin to hear her. Amu, Ran, Miki, and Su smile and leave. The next day Amu comes back as promised and greets Kotone, but she was still too focused on the violin to notice; her father tells Amu that Kotone had been like that the whole time, saying she had been going at it nonstop without even eating. Amu takes a seat behind the desk where Kotone is working and is amazed at how concentrated she was on her task, but agrees with Ran when she said it was kind of boring. While Kotone is reading a book to guide her along, Amu and her Guardian Characters are moving around trying to keep themselves interested in what was going on.
Kotone finally finishes and is pleased with herself. She must've known Amu was there though she hadn't really paid any attention to her, for she turns around to tell he she had finished, but sees that Amu had fallen asleep. Seeing her sleep, Kotone feels kind of sleepy herself and turns to her desk to lie her head down. When she did that, she moved her open book a little which knocked into her cup of tea that was set right next to it. The cup of tea falls and spills, running around the table and right into the violin. Amu awakes and sees the finished violin; going over to Kotone she tells her and sees her get up, but apologizes for waking her up. Kotone tells Amu it was no problem and picks up Saeko's violin to show Amu. That's when she noticed some tea dripping off of it onto the table. She wonders what happened and sees that she knocked over her tea, making the glue on the violin come undone.
Amu asks her what was the matter, but Kotone only says she didn't know what to do because Saeko was coming that day to get it. Amu suggests she ask her father, but Kotone says she can't because a mistake like that meant she wasn't meant to be a violin craftsman. They hear the phone in Kotone's father's office ring and hear him pick it up. He calls to Kotone saying that Saeko was on the phone for her, but she gets up and runs out of the shop, Amu behind her. Kotone's father opens the door to his office wondering where his daughter had just went. Amu loses Kotone somewhere trying to find her, but her Guardian Characters tell her they can sense Kotone's heart's egg. Miki says that it was feeling worse and worse, so Amu tells them they had to hurry.
In the park where Ikuto played his violin and Amu sang along with, Kotone is leaning up against a lamppost telling herself she could never become a craftsman, causing her heart's egg to become glazed over with gray before turning into an X-Egg. Amu comes running into the park and sees Kotone sitting on the grass against the lamppost clutching the violin. An X-Egg appears next to her, but flies off when Amu notices it. A giant cat paw comes out and catches it and Amu sees Ikuto, having Character Changed, in a tree now holding the egg. Yoru tells him that since it wasn't the Embryo he should just throw it away, but Amu tells him not to for it wasn't trash, it was Kotone's heart's egg. When Amu says Kotone's name, Ikuto looks at her to see if it were true, but that lead the X-Egg to get away and hatch into an X-Character. Ikuto looks around and finds Kotone, he looking somewhat surprised. Amu calls to Miki and unlocks her heart, becoming Amulet Spade.
The X-Character uses its violin to send an attack at Amu, but she blocks it with her paintbrush. A transparent picture of Kotone appears next to the X-Character and says that she wasn't cut out to be a violin craftsman, saying she had no talent. Amu asks her what she was talking about, which gives the X-Character a chance to send a powerful enough attack to knock Amu off of her feet. Frantically, Ran and Su fly over to Yoru and Ikuto and ask them to help, but Yoru tells them that they were supposed to be enemies. Ran gets angry and calls Yoru a lazy jerk while Su cheers Amu on.
Kotone appears again saying that she may have been speaking confidently, but ended up screwing up. She says she could never show her face to her father or to Saeko. Amu asks her what was the problem screwing up once or twice and says Kotone had a dream Amu couldn't imagine since it took violins a long time (one or two hundred years according to Kotone) to produce their rightful sound. Kotone tries to say something, but Amu goes on saying that when Kotone had told her about her dream, Amu thought she was pretty cool and tells her that she was dazzling. Amu tells her she should always hold onto her dream and locks onto Kotone's negative heart before using Open Heart to purify the X-Character.
Kotone's Guardian Character thanks Amu and tells her she had faith in Kotone, saying she was sure Kotone would be make plenty of violins that people years from now will think sound beautiful. Then she goes back to her egg which enters Kotone, waking her up from her trance. Kotone looks up and sees Amu who tells her they had to hurry back to the workshop so they could fix the violin. Ran suggests that Amu could Character Transform with Su to use Remake Honey to fix the violin and Amu and Su agree, but Miki and Ikuto think that if they didn't let Kotone fix it herself there really was no point. Kotone comes over to Amu and tells her she was going to rush back to the shop and leaves.
In the workshop, Amu is sitting still in her seat while Kotone is working hard on the violin again. She hears footsteps and sees her father walking over to her. She apologizes and he tells her that violins were instruments that could break. He says that all craftsmen knew that and didn't focus on making an instrument that wouldn't break. Then he says that when they do break, it was the job of a craftsman to fix them, saying it was the job of the musician to perform and the craftsman to do maintenance. Kotone smiles at her dad while Amu smiles from her chair.
Later in the day, Saeko comes back for her two violins. She thanks Kotone's father for the one he fixed, saying it was wonderful and asks Kotone about the one she wanted her to do. Kotone looks at her sadly before handing her the case. Saeko looks at it confused and Kotone begins to apologize, telling her what had happened. She tells Saeko she tried to finish it, but hadn't had enough time. Saeko asks her if she tried to finish it by herself and if she had asked her dad for help, and Kotone replies with a yes for both. Then she tells Saeko since she wanted her to do it, she had wanted to finish it herself. Saeko's quiet for a bit before telling her that she was sure she was going to be a great craftsman, surprising Kotone. She says that even though it was damaged, Kotone had put her heart into it and that it would make a beautiful sound soon. Kotone's father agrees, so Kotone asks Saeko if she could take and keep the violin for awhile to fix it. Saeko gives her the permission, saying she understood and hands the violin back to her.
Amu runs up and congratulates Kotone, who thanks her. She tells Amu that she couldn't remember exactly what happened, but she was sure Amu had helped earlier that day. She then thinks it was a dream and mentions Ikuto, thinking he may have helped her, saying he was wonderful and that she loved him. Amu looks at her, then Kotone changes her mind, wondering what she was saying and slaps Amu on the back, laughing. Amu laughs with her once she straightens up.
- Kotone's father: Her father is the owner of a violin shop. He seems very close to his daughter as she did want to grow up to have his choice of job. She appreciates the advice he gives, evident when she fondly smiles at him when he told her that violins were instruments that could break.
- Ikuto Tsukiyomi: Because Ikuto was somewhat of a frequent visitor at Kotone's father's violin shop, she began to develop a crush on him. It is implied that he may have noticed, but doesn't say anything about it. Kotone gets jealous at Amu when she thought Amu was Ikuto's girlfriend. He is nice to her when he shows up and seems to care about her, shown when he found out that he had just caught her X-Egg and didn't break it even though Yoru told him to. He even asks Amu that if Kotone didn't fix the ruined violin herself if there was even a point to it when Amu thought about fixing it with Remake Honey.
- Saeko Honda: Saeko is a famous violinist who had been coming to Kotone's father's violin shop for a long time. Kotone seems to look up to her as she was eager to see her and happy when Saeko asked Kotone to repair her violin herself. Though she thought Saeko would be mad when she learned of her damaged violin, Saeko wasn't and knows that the violin will still make a wonderful sound when fixed. She even lets Kotone take it another day to repair it when asked.
- Amu Hinamori: Though they really didn't know each other for a long time, the two seem to get along after Kotone finds out that Amu was not Ikuto's girlfriend. Amu congratulates her when Kotone got her first job request and came back the next day to watch her fix it and stayed even though she and Ran admitted it was boring sitting around. Amu tells her that there was no problem with making a mistake when Kotone's heart's egg became an X-Character. She also tells Kotone that she thought she was cool and dazzling when she was describing her dream.