He appears after Kevin C. Cucumber tries to trick SpongeBob into falling into a hole with a queen jellyfish ship. SpongeBob and Kevin are shocked to see him in person, as they identified who he is. The king jellyfish takes notice of them and thinks that the ship is the real queen jellyfish and he chases it until he hits a billboard for Kevin's Ointment. It breaks and the king jellyfish realizes that it is a fake.
He then notices Kevin and gets mad at seeing him. The king jellyfish chases the Jellyspotters and SpongeBob into a cave. He notices that the group has gone into a cave; he puts on a football uniform and starts hitting the cave. Eventually, he stops and Kevin sends an anchovy, who, at first, says it is okay to leave, but then gets electrocuted and returns.
After the Jellyspotters give up, SpongeBob walks out. The king jellyfish gets ready to electrocute SpongeBob, who takes out a bottle of bubble soap and blows a bubble that looks like a pie. The king jellyfish cheerfully takes the bubble pie and leaves.
Role in video games
Battle for Bikini Bottom
Squidward gets electrocuted by some jellyfish after being chased by a Fodder and SpongeBob says that the only way to cure severe jellyfish wounds is to get some king jellyfish jelly to apply as a lotion to the stun areas. Squidward says that SpongeBob should scale Spork Mountain and fight the king jellyfish. SpongeBob finds the King jellyfish singing in a giant shower at the top of the mountain (humorously singing alongside the in-game theme of Jellyfish Fields), and the king jellyfish is angry at him for interrupting his shower. SpongeBob fights him and wins; the king jellyfish gives SpongeBob a jar of his jelly and bids him farewell via gathering a suitcase and leaves.
The mini-boss battle involves waiting for the king jellyfish, who is floating around the arena with a tractor-beam-like-field below him that will damage SpongeBob if he gets too close, to land on the ground and emit an electrical shockwave from his tentacles. SpongeBob must jump over the shockwave and also attack the king jellyfish with a Bubble Spin, though the Bubble Bowl or Cruise Bubble can be used when they are later obtained if this fight is done in a later part of a playthrough; the very latter move can even be used to hit king jellyfish while he's still floating if SpongeBob is in a safe position to do so.
Each time SpongeBob damages him, the king will float back up and away towards under the shower and above the goo pool. From there, he will lay out hostile jellyfish enemies that will roam around the arena as king jellyfish will repeat his attack cycle with unleashing one more shockwave dependent on his HP amount. At one HP left, he will lay out blur jellyfish enemies instead. If all the jellyfish are ignored throughout the fight, it's possible for them to take up too much space around the arena.
This boss fight is also one of the few that can be manually exited though the way SpongeBob entered in (as there is no actual obstacle blocking the gate by Bubble Buddy's position), and thus, revisited before ever fully winning it. Occasionally, the standard mini-boss battle music can sometimes be overridden by the Jellyfish Fields theme upon triggering the fight for the first time in each playthrough.
The Yellow Avenger
He is in the Kelp Forest being attacked by three little Dirty Bubbles. The Yellow Avenger has to bounce on nearby plants to reach the king and then must throw waterballs in order to knock the bubbles off of him. After hitting him with three water-balls, he leaves and gives the Yellow Avenger access to the forest's clearing.
Battle for Bikini Bottom - Rehydrated
He reappears in this game, reprising his role as the mini-boss at the end of Jellyfish Fields. The king jellyfish has also been redesigned; he appears slightly translucent, closer to his appearance in "I'm Your Biggest Fanatic", and parts of the intro cutscene now have him use his brush as an imitation-microphone during his singing. Depending on circumstances, the fight can now take place either in the noon or sunset/dawn.
While the fight functions the same with the transitions between king jellyfish's moves being sped up, the Cruise Bubble can no longer work on king jellyfish since he now has an electrical aura while floating that blocks out all damage, and the aura can only be dispelled when he unleashes his shockwave.
In the video game Battle for Bikini Bottom and its remake, SpongeBob accidentally disturbs the king jellyfish's shower during a request to obtain some jelly. He seems to have this anger in "I'm Your Biggest Fanatic." SpongeBob gives him a pie and they are thus on good terms.
Relationship with Kevin C. Cucumber
In "I'm Your Biggest Fanatic," it appears that the king jellyfish is familiar with Kevin as he recognizes him and gets angry with him for something that he may have done to him. He then chases him, SpongeBob, and the other Jellyspotters.
King Jellyfish's Latin name is Cnidaria Rex. Cnidaria is the phylum of mostly aquatic marine invertebrates that have cnidocyte, a specialized stinging cell found in creatures such as jellyfish and sea anemone. Rex is Latin for the king.
Unlike other jellyfish, he actually talks, which can be heard briefly when Kevin and his members go inside the cave.