Plankton

"Hear me, Krabs. When I discover the formula for Krabby Patties, I'll run you out of business. I WENT TO COLLEGE!"

- Plankton's first quote.

Sheldon James Plankton, better known as Plankton, is the main antagonist of Nickelodeon's popular cartoon series Spongebob Squarepants. He is an evil zooplankton and the business rival and personal archenemy of Mr. Krabs, the owner of the Krusty Krab where SpongeBob Squarepants works. Despite being appearing as antagonist, sometimes he appears as an anti-hero.

While the Krusty Krab is popular among the sea creatures of Bikini Bottom, Plankton's own restaurant, the Chum Bucket, gets little to no customers. Plankton's plans all revolve around stealing the elusive "Krabby Patty Secret Formula" so he can copy it and make the Chum Bucket popular. He lives inside his restaurant, where he has an elaborate laboratory and a computer/AI "W.I.F.E." (Wired Integrated Female Electroencephlagraph) named "Karen".

He is voiced by Doug Lawrence and Sirena Irwin (as Cashina).

Series
Plankton constantly attempts to steal the formula for the Krabby Patty, the delicious burger-like food that is sold in the Krusty Krab. Plankton believes that the Krabby Patty is the reason for the Krusty Krab's success, and that his Chum Bucket would be equally popular if it sold the Patties as well. However, ensuring the success of his business is only the first step in Plankton's plan, which will culminate in him ruling the ocean.

For the events of post-movie seasons (namely season 4 and so on but set in events between first 3 seasons and the movie), Plankton do find alternate means to success his business albeit only temporary. Stealing Krabby Patty however, still his main priority however.

Movies
"Oh grow up! What, do you think this is a game of kickball on the playground? You never had a chance to defeat me, fool! And you know why? (SpongeBob: Because you cheated?) No! Not because I cheated! Because I'm an evil genius... and you're just a kid! Haha! A stupid kid! HaHaHaHaHa!"

- Plankton to SpongeBob in the movie.

In The Spongebob Squarepants Movie, Plankton serves as the main antagonist of the film, and it marks the first time when he becomes an alarmingly dangerous and depraved villain of the franchise. He finally manages to steal the formula from Krabs by framing the latter for the theft of King Neptune's crown, resulting Krab to be frozen in ice for 6 days. This allows the Chum Bucket to become just as popular as the Krusty Krab had been previously. Plankton even comes close to his eventual goal of ruling the sea, by giving away mind-controlling bucket hats free with his Krabby Patties, turning Bikini Bottom into "Planktopolis."

Plankton also hires a hitman named Dennis into stopping SpongeBob and Patrick when they try to retrieve the crown to clear Krab's name. Even when SpongeBob and Patrick finally manage to defeat Dennis and retrieve the crown from the Cyclops, this still doesn't stop Plankton as he puts a mind-controlling helmet on Neptune, revealing that the theft of the crown was nothing more than a ruse to get Neptune under his control and take over the seas for himself. Eventually, Plankton is eventually defeated by Spongebob when he sings the "Goofy Goober Rock" to remove the bucket helmets from all the fish. Plankton is then arrested for his crimes and sent to jail.

Sponge Out of Water
Plankton returns in the 2015 sequel The SpongeBob Movie: Sponge Out of Water as the secondary antagonist turned anti-hero. He attempts to steal the Krabby Patty secret formula yet again, but then it disappears when Burger-Beard writes in the magic book to steal it. Then he later becomes an anti-hero, when he begrudingly teams up with Spongebob who was rescuing him from Mr. Krabs and the people of Bikini Bottom falsely accuse Plankton for stealing the formula and Spongebob for "joining his side". Throughout the journey, Spongebob tries to show Plankton the meaning of teamwork, much to his annoyance. Then they go time traveling to keep the secret formula from disappearing. Then Plankton, who was hiding inside SpongeBob's pocket, surprisingly appears to help out with Spongebob, Patrick, and the others to get the Krabby Patty secret formula back from Burger-Beard (the true culprit) as super heroes.

Using the magical book that Burger-Beard stole earlier, Plankton becomes a giant muscle-bound hero (dubbing himself as Plank-Ton). After Burger-Beard defeats SpongeBob, Sandy, Plankton, Squidward, and Krabs, he attempts to escape, only for the muscle-bound Plankton to stop him by lifting up his ship. Embellished with the true meaning of teamwork, Plankton helps SpongeBob create a storm full to bubble to capture Burger-Beard and force him to return the formula.

After kicking Burger-Beard away into a marooned island, Plankton ponders on what to do with the formula, much to Krabs' distraught. However, Plankton decides to give the formula back to Krabs, admitting that keeping things to himself is a selfish thing to do. As everything in Bikini Bottom was turned back to normal, Plankton continues his role as the Krusty Krab's business rival going back to his formula-stealing tricks just to keep things in the status quo.

Battle For Bikini Bottom
Plankton appears in the 2003 video game. He was originally thought to be it's main antagonist, but that was to hide the game's real main villain.

In this game, Plankton creates an army of robots to take over Bikini Bottom, but forgets to make them obey him, causing his creations to run amok across the sea bottom. Plankton uses SpongeBob, who is convinced that he caused the robot invasion while role-playing with Patrick, to get rid of the robots, which are led by a Robotic Plankton.

Nicktoons Unite!
Plankton is one of the villains who compose The Evil Syndicate (along with Professor Calamitous, Denzel Quincy Crocker and Vlad Plasmius), the main antagonists of the game. He was charged to harvest energy out of the jellyfishes in Jellyfish Fields for the Doomsday Machine the Syndicate was building, making changes in the Chum Bucket to make it a factory. He also began to kidnap citizens of Bikini Bottom (including Patrick) who didn't go along with his plans. However, the group of heroes assembled by Jimmy Neutron, Spongebob included, managed to free the jellyfishes and defeat Plankton at the Chum Bucket. He retreated to Retroville were he tried to destroy the heroes along with his fellow Syndicate members. But even that failed as his shield was turned off and he was defeated again by Spongebob.

Nicktoons: Globs of Doom
Plankton returns in this game as the only original member to be part of the second Syndicate. He, along with other villains, join forces with the heroes to gather parts for the Vessel of Portentia and defeat the Morphoids. However, he also betrays the heroes at the end when they are about to defeat Globulous Maximus and the villains steal the Vessel. However, he and the others were defeated when Maximus takes them one-on-one at the moon, destroying the Vessel and leaving them in the vacuum of space.

Personality
Plankton is an evil mastermind through and through. His plans always involve technological devices and weapons, all for the sake of the formula. His main goal has always been throwing the Krusty Krab out of business. In Seasons 1 to 5, he was completely unable to get any success for his Chum Bucket restaurant (as majorities of his schemes involving attempted theft on Krabby Patty recipe). However, in both events of the first movie and later seasons, he was now able to get more chances of success other than stealing the Krabby Patty formula, such as having an attractive restaurant place where the people will come to observe the underground complex beneath both his and Krabs's restaurant. Other examples are putting slogans such as "Chum is Fum" and "Fum is Chum" (where Patrick Star helped him in his slogans until the starfish's both low IQ and literally overheated brain became the setbacks) and one person named Nat Peterson kept coming to buy chum to eat, although he was only doing it since Karen paid him to do so.

All in all, Plankton's small stature, the fact pretty much everyone else in his life looks down on him, and his poverty all cause Plankton to feel very insecure, and even defensive. He clearly suffers Napoleon syndrome, and his desire for conquest stems from his deep-seated desire for respect, and even admiration, both of which are absent from his life. From his childhood of poverty, all the way to his adulthood of feeling neglected, and downtrodden, his bitterness and hatred of nearly everyone has grown, and consumed him, almost molding him into a fully-fledged psychopath. However, traces of empathy are to be found, as he truly loved spot, the amoeaba, and even though he seems to look down on, and belittle Karen, he does actually love her, and Karen returns this love although she is often snarky, much to Plankton's frustration.

Also, it is worth mentioning, due to his small size, and the fact he tends to have huge flaws in all his plans, no-one in Bikini Bottom really sees Plankton as a threat (when Sandy managed to capture Plankton for stealing her fur, the police arrested her instead out of her unintentional public nudity and spared Plankton a jail sentence, being too oblivious of Plankton's existence or his scheme) and is rare for the other notice him when he is up to something. Most people merely see him as a pest, though Mr Krabs acknowledges that if he DID get the the formula, he would make the chum bucket a bigger risk to the Krusty krab, so Mr Krabs is nonetheless vigilant around plankton though this doesn't stop Mr Krabs from mocking plankton. The only person who is truly afraid of Plankton and his outlandish schemes is SpongeBob, especially since the events of his debut episode where he used a mind control on the sponge and nearly discovered the Krabby Patty's formula in process (whilst being ended in failure, this was enough to alarm SpongeBob about Plankton's villainous qualities and dangers that he can pose to the others). While Plankton often appears as a bumbling comic foil, he actually can be a real threat under certain circumstances, manipulating others to get his way, and when the odds are in his favour, he can be surprisingly successful, though this success is usually short-lived.

Trivia

 * Plankton found Mr. Krab's' life to be worse than his life, when he experienced it in The Algae's Always Greener and Spy Buddies. In The Algae's Always Greener, he found lots of reasons that make Mr. Krab's life difficult:
 * SpongeBob is always asking him for a Weekly Performance Review on Tuesday and makes a teensy little temper tantrum if Plankton refuses. He also keeps yelling "SOILED IT! SOILED IT! SOILED IT!" when he accidentally gave a customer a large soda when the customer wanted a medium soda.
 * Squidward, the lazy, grumpy, pessimistic worker at the restaurant, complains to Plankton that he doesn't want to work with SpongeBob. Later, when Plankton put him in charge of fry cooking, a customer got angry because the Krabby Patty that Squidward cooked was very ugly, disgusting-looking, and hideous.
 * Pearl, his virtual reality daughter, throws a big temper tantrum because she doesn't get more than $1.
 * Krabs tries to steal the Krabby Patty formula (which Plankton actually does in the show's reality), and Krabs NEVER gives up like he does.
 * In the first movie (The SpongeBob SquarePants Movie), Plankton was a lot more evil and depraved than he was in the series as he ultimately grown into alarmingly dangerous threat to the entire ocean. In the said movie, he stole King Neptune's crown, frames Mr. Krabs for it, enslaves Bikini Bottom (which also includes not allowing his slaves to earn a single rest or break and just work nonstop), shows excitement and joy when Mr. Krabs is going to be killed, and sends Dennis to kill SpongeBob and Patrick rather than merely get rid of them. He also enslaved King Neptune, in which gave him a potentially powerful asset in conquer the whole ocean and the world. This is likely the only time Plankton came close to being a complete monster, making it his most evil moment in the whole franchise.
 * This however, ultimately outclassed by Burger-Beard in the sequel, as aside managed to instantly steal the Krabby Patty recipe simply by rewriting the magic book that records everything that occurs in the ocean (especially events in Bikini Bottom includes conflict between Plankton and Krabs), the pirate was able to destroy Bikini Bottom and render it into post-apocalypse state in an instant. Even in order to defeat Burger Beard, it took the superpowered SpongeBob, Sandy, Patrick, Krabs, Squidward, and Plankton himself to defeat him.
 * Early in the series, Plankton is considered the primary villain of the series. Later, as Krabs becomes more dramatically greedy (which the said greed ultimately drove him insane) and Plankton becomes less dramatically evil, the role of the main villain of the series becomes more debatable as lots of people think Mr. Krabs is evil. Plankton also acknowledge this himself where he began to question which he or Krabs that truly evil.