Kevin the Sea Cucumber is the leader of the Jelly Spotters, and the main antagonist of the episode "I'm Your Biggest Fanatic" who is voiced by Dee Bradley Baker. He was speaking at a jellyfishing convention, in which SpongeBob and Patrick attended. SpongeBob was a big fan of him, and would do anything Kevin told him to do. One day, he invited him to go jellyfishing with the Jelly Spotters. He tricked SpongeBob into thinking he could join their club if he passed certain tests, simply so that he could humiliate and belittle him. This seems to be a habit of his, as many of his biggest fans have ended up stranded in a crevasse at Jellyfish Fields because of him.
However, all of Kevin's plans ended up with him getting stung by jellyfish one too many times, causing his hatred for SpongeBob to grow. Finally he snapped and said SpongeBob was ready for the final test: use SpongeBob as live bait to catch a queen jellyfish.
It was revealed the Jelly Spotters were using a robot queen jellyfish. Kevin and his friends then taunted and made fun of SpongeBob so badly by calling him a "loser," but when a king jellyfish fell in love with the robotic queen and attacked them, Kevin proved to be a coward and admitted that he was only in the club for the fashion. When SpongeBob managed to ward away the king, Kevin was ruined when the Jelly Spotters ripped off his crown-like cranium and put it on SpongeBob. Although SpongeBob left the club, it's unlikely Kevin retained leadership of the club, as he lost the loyalty of his teammates. Since then, Kevin hasn't been in any other episode.
He doesn't seem to care about the well being of his fans, as he didn't care about SpongeBob punching himself, twice, or falling off a building.
Kevin, nor the Jelly Spotters, have not been seen in any other episode most likely because SpongeBob spread the word and the Jellyspotters lost their reputation PLUS they didn't have a leader anymore.