Marimo's heart's egg was turned into an X-Egg when Marimo made a mistake while playing the piano at practice for Seiyo Academy's students to sing along with for the 23rd Choral Competition. Saaya and her group come along and say mean things to her because Marimo got the part of playing the accompaniment and Saaya hadn't. Marimo stands from the piano and tells Tadase she was okay when he called to her and says that she thought someone else should play the piano and runs off from the practice. Marimo and her Guardian Character appear in Episode 045.
Marimo's Guardian Character is shown with blonde hair and a pink eighth note on the left side of her hair. Some of her hair is pulled up into a ponytail and the ends of the ponytail are pulled around so that they're on either side of her head. She wears an orange dress with a white collar and white cuffs. Though hard to see, her shoes seem to be either pink or orange. Coming from the middle of her collar is a scarf or tie decorated to look like piano keys. Her eyes are brown and she doesn't mention her name to Amu.As an X-Character, she looks like a typical one, black with a red "X" on their foreheads, but she has a baton which she uses to send most of her attacks.
When Marimo auditioned for the job of playing the accompaniment on the piano for the competition, she won it, much to Saaya's anger because she had also auditioned. When Saaya and her friends notice her and Seiichiro on a bridge talking to each other late in the day, she starts to look content and one of her friends said they had a feeling she was planning something. The next day at practice, Tadase tells the students that since the competition was the following day, their current practice would be their last. He begins to conduct, Marimo plays the piano and the students start to sing the song. They stop when Marimo messes up, so she apologizes and asks if they could start over. He tells her they could, but someone not on stage says they couldn't. Looking back, they see Saaya and her friends standing there. The five begin to say mean things about Marimo's playing and talk about winning, but Tadase tells them they would be alright and tries to get everyone to start singing again, but Saaya and her friends go on.
After a little bit, they all hear a noise and see that Marimo had stood from the piano and was looking down. Tadase calls to her, but she says she was alright and that someone else should play the piano before she runs from practice. Amu follows after and catches up to her and tells her she thought Marimo should play the piano because she was the better choice. Marimo thanks her, but says she thought someone else more skilled should play before running off again. Amu calls after her, saying she hoped she would come to the competition and that everyone would wait for her.
When some of Seiyo Academy's students are at the 23rd Choral Competition, Marimo is elsewhere standing in front of a bench wondering if she should go to the competition. She remembers some of the things Saaya had said and looks down sadly while her heart's egg gets glazed over with gray before turning into an X-Egg. Amu's Guardian Characters tell her that an X-Egg had been born when Seiichiro comes up to the Guardians to tell them that he knew Marimo would be there and that he was going to go look for her. Amu and the Guardians run after him.
Seiichiro finds Marimo sitting on a bench and hears her X-Character, which had just hatched, laugh, but thinks he was hearing things. Her X-Character yells "forte!" and aims an attack at Seiichiro using its baton, causing him to fly back some and hit the ground, losing his glasses in the process.
Marimo says that she was wrong, and Seiichiro hears her, looking back in her general direction. Yaya runs over and tells Amu she found the Marimo and Seiichiro. Amu sees that her X-Egg already hatched into an X-Character, so Tadase tells them they should go stop it, but Amu stops him when she sees Seiichiro pick himself off the ground. He tells Marimo that she wasn't wrong and Yaya thinks he couldn't see the X-Character without his glasses. He goes on saying that she had told him her dream of wanting to play the piano always and a transparent picture of Marimo appears next to her X-Character saying that everyone said there was no point unless you were the best and calls her dream worthless. Seiichiro tells her there was no thing as a worthless dream and she gasps. He tells her that he was the same as her because he wasn't good at a lot of things, but he still had dreams and says that he wanted to become president of a company someday, an astronaut, and a race car driver. Marimo calls him stupid and says there was no point in having dreams like the ones he had and her X-Character fires another attack at him.
Tadase tells Amu that it was becoming dangerous, but Seiichiro is still going, saying that there may have been no point, but that his dreams were all he had to hold onto at the moment. He slowly starts to approach Marimo the best he could because of the Character's attack and says that if he began to think his dreams were worthless, he would become pathetic. He says that his dreams were his and his alone and that his dreams were all that mattered, causing the X-Character to stop its attack. Amu steps forward and agrees with Seiichiro. Seiichiro still can't see and calls to Amu who tells him he did a good job before she unlocks her heart and does a Character Transformation with Ran, becoming Amulet Heart.
Marimo's X-Character tries another attack, but Amu jumps to dodges and it hits Seiichiro instead, making him fall back again, and Amu apologizes to him. She goes at the character from a different angle and it tries its attack again, but she uses her Pom-poms to block it and ends up right next to it. She tells Marimo's X-Character that she liked her piano playing and then uses Open Heart to purify it. Marimo's Guardian Character appears and thanks Amu and Seiichiro, but he doesn't hear as he was still on the ground looking for his glasses. She goes back to her egg just as Seiichiro finds his glasses and puts them on. Marimo wakes up from her trance and wonders what she was doing when Yaya tells her everything would be fine. Tadase steps toward her and asks if she would play the accompaniment for them and she gladly accepts.