“When Will My Life Begin?” in TANGLED (2010) does such a great job introducing Rapunzel. She spends the entire song painting, baking, reading, and filling every hour of the day, yet you can feel how desperately she wants something beyond the tower.
Edna Mode in THE INCREDIBLES (2004) has maybe ten minutes of screen time and somehow leaves with half the movie. A character so entertaining that an entire generation can hear “No capes!” in her voice immediately.