Shugo Chara! - Season 1 Episode 45

During attendance day, the Guardians announce that the school will be participating in a choir contest. Amu ends up with the lead part while Marimo is chosen as the pianist. Because Seiichiro is a bad singer, Marimo helps him practice by playing the piano with him, and tells him about her dream to keep playing the piano. During rehearsal, Marimo makes a simple mistake and Saaya and her group criticize Marimo for it. Marimo becomes upset and excuses herself from the rehearsal. The next day during the competition, Marimo does not show up. Because of Saaya's earlier criticism, Marimo's Heart's Egg turns into an X Egg and hatches. The Guardian Characters sense this and the Guardians, along with Seiichiro, go looking for Marimo. With Seiichiro's unwitting help, Amu transforms into Amulet Heart and purifies the X Character. The choir performs successfully, and Amu Character Changes to sing her solo.