The Dark Knight is a squirrel that only appears in the episode "Dunces and Dragons." She works for Plankton's ancestor, Lord 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 it and SpongeBob and Patrick save Pearl from being dropped into the lava.
The Dark Knight looks the same as Sandy 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.
[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!
It is revealed when she is defeated that she then had a passion for karate, and that she loves it more since SpongeBob and Patrick went back to present time.
Unlike Sandy, she is not wearing an air helmet underwater which does not make sense at all, because the Dark Knight is a land creature. It is possible that Planktonamor's magic keeps her from suffocating.