Shugo Chara! Season 1 Episode 45 Episode 45 - Go for it! Seiichiro!

"During attendance day, the Guardians announce that the school will be participating in a choir contest. Amu ends up with the lead part while Marino is chosen as the pianist. Because Seiichiro is a bad singer, Marino helps him practice by playing the piano with him, and tells him about her dream to keep playing the piano. During rehearsal, Marino makes a simple mistake and Saaya and her group criticize Marino for it. Marino becomes upset and excuses herself from the rehearsal. The next day during the competition, Marino does not show up. Because of Saaya's earlier criticism, Marino'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 Marino. 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."