King Neptune, born 5,000 years ago, is the series' depiction of Neptune, the Roman god of the sea. In the universe of SpongeBob SquarePants, Neptune is the god and supreme ruler of the sea. His name is often used in place of God's, with phrases such as "Dear Neptune," "Neptune preserve her!," and "Oh my Neptune!" However, he has never explicitly been shown to be worshiped by the creatures of the sea.
In The SpongeBob SquarePants Movie, he tried to kill Mr. Krabs when he was falsely accused of stealing his crown, with no evidence that Krabs stole it aside from a false call (made by Sheldon J. Plankton). He also kept Mr. Krabs frozen for days and nearly killed him after having his stalling daughter left.
In "Neptune's Spatula," he keeps thinking SpongeBob is just a joke, and can't pull out the spatula. He then becomes destructive and repeatedly burns Patrick, only to fix him later, albeit incorrectly with his face on his butt.
King Neptune first appears in the season 1 episode "Neptune's Spatula," where he spawns after SpongeBob pulls the Golden Spatula from the ancient grease, making him Neptune's royal fry cook. However, Neptune refuses to accept such a lowly creature like SpongeBob as the chosen one and challenges him to a cook-off where, if the latter wins, he will become a god in Atlantis, but if he loses, he must give up fry cooking forever. The challenge is held at the "Poseidome," where the first one to make 1,000 patties wins. Using his godlike powers, Neptune wins the challenge just as SpongeBob finishes his first patty. To celebrate his victory, he gives the patties to the audience, who hate it. Neptune then tastes SpongeBob's patty and loves it. He declares SpongeBob the winner and makes him a god. However, when SpongeBob learns that his friends can't join him in Atlantis, he does not want to go and instead, makes Neptune his "trainee" at the Krusty Krab. In this episode, Neptune is green and has fairly realistic human features, has hair, a mustache, a beard, thick fingers, is very muscular, and has a different crown. This version of Neptune also appears in Battle for Bikini Bottom, where he summons SpongeBob and Patrick to the Poseidome to defeat a robot version of Sandy.
Neptune plays a central part in The SpongeBob SquarePants Movie. The movie also introduces Neptune's daughter, Princess Mindy. This Neptune is actually the same person who was haircut, green-skinned and far more cartoony than the version in "Neptune's Spatula." He is also explicitly portrayed as a king rather than a god and lives in a palace just outside of Bikini Bottom rather than Atlantis. This Neptune is very brutish and tyrannical, which he tells his daughter is necessary for him to be respected as a ruler. His crown is very important to him, not only because it is a symbol of his power, but because it covers his "thinning" (bald) head. When the crown is stolen by Plankton, Neptune attempts to execute Mr. Krabs, who is framed for the crime. In "SpongeHenge," Shubie denies Neptune's existence when an anchovy tells her that Neptune must be mad, saying, "Everyone knows Poseidon is the ruler of the undersea."
A picture of King Neptune, as seen in "Neptune's Spatula," is shown in "Krusty Towers." Squidward refers to this Neptune as "Neptune XIV," suggesting that there are multiple King Neptunes and possibly explaining his inconsistent appearance.
In "Trident Trouble," the Neptune's Spatula version of King Neptune appears. SpongeBob accidentally gets ahold of his trident and Neptune gets SpongeBob's spatula, causing trouble.
In "High Sea Diving," the Neptune's Spatula version of King Neptune reappears. At the very end of the episode, he was angry at Old Man Jenkins for tricking the town with his made-up stories. As punishment, Neptune zaps the deluded old man with his trident.
Judging by his different appearances, it is reasonable to conclude that Neptune is actually various different characters, with the first one referring to the Roman deity of the same name, and while a second one works as the actual King of SpongeBob's underwater world. Their different places of living, the fact that the King didn't already know SpongeBob, and his daughter Princess Mindy only being in the movie reinforces this theory. There is also the fact that in "Krusty Towers" Squidward says "King Neptune XIV would be nice." Which claims that the three versions of Neptune throughout the series are not the same character and instead could just be different monarchs of the royal family and are probably related.
Another possible explanation is that he has gotten older from the show to the movie resulting in his loss of hair, because Stephen Hillenburg did establish that the movie does take place after the series. However this is hard to say seeing as the first movie takes place 36 years after the pilot episode (because of SpongeBob's 374 straight employee of the month awards), and so far Neptune (from "Neptune's Spatula") has been relieved to have been around for 5,000 years, meaning that 36 years (or less as his 5,000th birthday took place after SpongeBob got his job) probably won't age him that much. Plus there is the fact that he is a god (hence immortality) so he might not age at all once he reached a certain age point.
Employee of the Month - An amusement park called Neptune's Paradise is a destination for SpongeBob and Patrick.
Battle for Bikini Bottom - King Neptune invites SpongeBob and Patrick to the Poseidome to face off against a giant Robo-Sandy which they manage to defeat.
The SpongeBob SquarePants Movie (video game)- King Neptune's role is the same as in the Movie: his crown goes missing and he accuses Mr. Krabs for taking it. He later sends SpongeBob and Patrick to find it, allowing Plankton to take Bikini Bottom for himself. His crown is returned and he frees Mr. Krabs. Under the control of Plankton, he is the final boss of the game.
SpongeBob's Atlantis SquarePantis - King Neptune makes a cameo in a painting in the Bikini Bottom Museum as an object SpongeBob must photograph. The painting is also unlockable in the gift shop with The Sirens added in it.
Clash of Triton - Triton, King Neptune's son, captures King Neptune on his birthday at the Krusty Krab. After his and Queen Amphitrite's tridents fall near SpongeBob and Patrick they gain powers and defeat Triton's underlings (The Monsters from the Party). They defeat Triton, restore Bikini Bottom to its former glory, and reunite Triton with his proud father.
"This joke has gone far enough!"
"I think I want to try it for a second time!"
"Quit stalling this execution!"
"Go wait in the carriage!"
"I'm not bald! It's thinning!"
"My crown is in the forbidden Shell City?!""
"Eugene Krabs, your six-day reprieve is up and it is time for you to die!"
"And once your little champion fails to return, I get to splatter this crab all over the walls!"
"My hair's thinning a bit."
"You, go to Shell City? (evilly laughs)"
"No one who's gone to Shell City has ever returned! What makes you think you could? You're just a kid."
"Very well then... I'll have to fry you both!"
'Oh, I'm sorry for falsely freezing you, Krabs, and may I say, sir. you are a very lucky fellow to have your own employ such a brave, faithful and heroic young lad.'
"Squire, will you hurry?... (Squire accidentally sprays hair in his eyes and screams)
"Except give SpongeBob and Patrick a little more... Time-time-time-time-time. What?"
"It looks like I just hit a single! Let's see if you can get a home run, son!"
"Oh, how I miss my son!"
"Son, I demand to know the meaning of this impudent imprisonment!"
"You're going to be a god and like it!"
"Maybe we do have a problem..."
"Gah! Where am I, in Crazytown? I have had enough of this nonsense!"