The Quickster is a superhero who is only seen in the episode: Mermaid Man and Barnacle Boy V. He is the part of The International Justice League of Super Acquaintances. It can be assumed that he retired and is replaced by SpongeBob in this episode. He has the ability (as his name describes) to run extremely fast. His costume is white with a red lightning bolt on it.
There is a game on Nick.com which The Quickster has to avoid lava and is called "The Quickster".