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 worked for Plankton's ancestor, Planktonamor for a while, until SpongeBob defeated her. She then joined SpongeBob, Patrick, and Squidly come along. She decides to switch sides because SpongeBob is very good at Karate.
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.
[male voice] HALT! Who goes there? [to SpongeBob, Squidly, and Patrick]
[male voice] I ask once more. Before I rip you limb from limb, reveal thyself!
[female voice] Rrrrrrrriippppp!
When she is defeated, it is revealed that she found SpongeBob's karate fascinating. It is suggested that because of this, she taught her child this, who then passed down the skill until it reached Sandy Cheeks.
Unlike Sandy Cheeks, she is not wearing an air helmet underwater which does not make sense because the Dark Knight is a land creature.
It is very possible that Planktonamor's magic kept her from suffocating.