Cappie is the younger sister of Hattie. She first appears in Keeping Cappie Happy. Hattie refers to her as her "baby" sister while the official description of the episode describes her as a toddler.
She is too young to speak, and actually casts a spell before being able to form a sentence. By saying "goo goo ga ga" she magically creates goo which covers Bobgoblin, who says he loves babies.