I'm somehow feeling Sadness atm because I am suffering a bout of disappointment, and it's one of those things that I let sink in too much and question if I really am a failure at things because many goals I had often ended in failure. I often allude it to a cool kids' club and me not being cool enough to get in.
I know how much I can relate to Sadness, because she often gets stopped by Joy, even at times when the situation called for Riley to be sad (along with Joy trying to stop a blue core memory produced by Sadness, leading to both of them going on the journey through the other areas and meeting Bing Bong). Joy pretty much thought Sadness wasn't important, but the climax then reveals that she is just as important as the others. Plus, if you watch the scene with Riley's parents' emotions, Sadness is leading her mother's emotions (Joy I think was the number two for her) and Anger is leading her father's, with Fear being the second-in-command.
Things got very dark for Riley when Joy and Sadness were sucked out of headquarters and did that long journey, because it left Anger, Fear and Disgust controlling Riley and she wanted to run away in an attempt to want to go back to Minnesota. You could clearly tell during that dark time of the film just how much Riley's personality depended on Joy, but also Sadness.