“The emotion nobody wanted around — until it turned out empathy and connection are impossible without her.”
She looks like a hindrance for the first half of the film — and that's the point. Sadness is the prerequisite for empathy and the key that unlocks genuine connection. She doubts her own value throughout, yet that very quiet passivity is what comforts Bing Bong and opens the only exit when Riley finally breaks. Slow and heavy, but the most human emotion of all.