The Dark Knight looks the same as Sandy Cheeks and she is very tall, but she is revealed to be Sandy's size when she is defeated. She wears silver, orange, and yellow medieval armor. The Dark Knight also carries around a weapon known as a flail.
She goes into the castle to save Princess Pearl by telling the guards that she was going to torture SpongeBob, Patrick, and Squidly but just went up to save Princess Pearl. When the jellyfish dragon comes up, The Dark Knight gets zapped by the dragon, and SpongeBob and Patrick save Pearl from being dropped into the lava.
Unlike Sandy, she is not seen wearing an air helmet underwater.