Mozart acts as the Music Room guardian at St. Claire Elementary, manifesting as a living portrait beside fellow composers Bach, Beethoven, and Schubert. He evaluates participants pursuing the Music Room medal during the After School Midnight Party challenge. Proud of his musical legacy, he reacts with visible anger—marked by blank white eyes—when characters recognize Beethoven's work more readily than his own. Despite this pride, he exhibits openness to unconventional musical expression, enthusiastically accepting Mutsuko's single, forcefully struck piano note as a valid minimalist composition. Alongside the other composers, he deems this innovation sufficient to award the medal.

Described as a "Cloud Cuckoolander," Mozart displays a whimsical and unfocused personality, aligning with his status as the youngest composer guardian. His reactions and behavior contribute to the Music Room challenge sequence's chaotic yet structured nature.