In the episode "Sworn to the Sword" Pearl relates herself to Connie with wanting to protect Rose/Steven. In her song she sings "deep down you know you weren't built for fighting" this suggests that Pearls are manufactured to serve the Diamonds as servants. Jasper called Pearl "a lost defective Pearl" because she thought that Rose was gone. Without Rose, Pearl has no purpose thus making her lost. She is operating without a master so that makes her defective.