Sheldon J.Plankton, more commonly known only as Plankton, is a green planktonic copepod who is Mr. Krabs' business rival. He runs an unsuccessful restaurant called the Chum Bucket alongside Karen, a waterproof supercomputer who is his sidekick and wife. The Chum Bucket is extremely unpopular in Bikini Bottom because it serves chum, a type of bait made from fish meat.
Both Plankton and Karen are the two main villains of the franchise. Plankton despises Mr. Krabs for running a successful restaurant because his own business rarely gets any customers. His primary goal is to steal Krabs' secret formula for Krabby Patties. Karen gives him a variety of evil plans to steal it, but their efforts always end up failing in the process.
While Plankton is usually portrayed as an evil character, he is sometimes shown as a sympathetic antihero in episodes that focus on him. He is voiced by Mr. Lawrence, one of the show's staff writers. Stephen Hillenburg originally planned for both Plankton and Karen to be one-time minor characters, but Lawrence wrote them into many episodes and proved that they could work as the main villains. Both Plankton and Karen first appear in the episode "Plankton!"
Plankton succeeds at stealing the formula in the movie, but his goal of world domination is eventually thwarted by SpongeBob. In "Friend or Foe," it is revealed that Mr. Krabs and Plankton were best friends during their childhood, but a dispute over the Krabby Patty secret formula ended their friendship for real, meaning that Plankton had a part in the creation of the Krabby Patty.
Plankton is, as his name suggests, a tiny dark green-colored plankton who can fit in the palm of one's hand. He has some stubby appendages and one yellow eye with a red pupil. He has two long, skinny antennae with four small green spikes sticking out along them. He also has one black thick eyebrow. In some episodes, he is shown without his skin, like in "Plankton's Regular" and "Single Cell Anniversary." His size constantly changes: in some episodes, he's the size of a Krabby Patty, meanwhile in others, he can perfectly hide in a Krabby Patty.
When they were kids, Plankton and Krabs were considered outcasts by other kids, most notably a school group led by Billy because Krabs was poor and Plankton was already very intelligent, which earned him the image of a nerd. When they discovered that money could bring them out of their poverty, they decided to make money selling hamburger patties and set up a mini-restaurant in the local dump.
At first, their burgers were not successful; the first of their patties were tried by Old Man Jenkins, who was knocked out by the contaminated burger. Plankton started complaining about how Jenkins had been old and not able to handle the patty. Offended by this, Krabs tried to take the recipe from Plankton to improve upon. Plankton wanted to do this himself and both him and Krabs argued, but he only ended up with the corner of the recipe, which read: "And a pinch of chum."
Plankton stormed out of the restaurant and went to create his own recipe, using only his memory, the corner of the recipe, and science. At school, Plankton tried to sell his chumburgers in a chum bucket and failed, while Krabs succeeded with his Krabby Patties, which he had created by accident when Plankton had slammed the door. Ingredients fell in the patty batter, thus creating the patty. Plankton later named his restaurant the Chum Bucket, after the secret ingredient of the original unsuccessful patty recipe. Mr. Krabs named his restaurant the Krusty Krab, after himself.
To this very day, Plankton has been single-mindedly obsessed with obtaining the Krabby Patty formula and running the Krusty Krab out of business. He often goes to incredible and quite ridiculous lengths to do so. In "SpongeBob's Last Stand," for example, he builds a massive highway that goes straight through Jellyfish Fields and over the Krusty Krab, leading right to his restaurant.
Even in the present day of the series, Krabs and Plankton are occasionally shown to get along somewhat, such as in "New Leaf" and "Best Frenemies."
The major on Plankton's degree is Tank Driving. He's seen driving a tank in "Teacher's Pests" as well.
Later, when Plankton was an adult, he married Karen in 1992. He is very ashamed of his first name, Sheldon, and gets embarrassed when Karen makes fun of it in "Plankton's Army." However, she calls him Sheldon in many other episodes and he does not seem to mind. It is revealed in "Walking the Plankton," that despite the fact that he's married to Karen, they never had a honeymoon.
Plankton's restaurant, the Chum Bucket, has always been extremely unsuccessful, not only because of competition from the Krusty Krab and Plankton's rude and cruel treatment of the people in Bikini Bottom, but also due to the extremely poor quality of the food and Plankton's constant failure to clean the place up. As its name implies, the Chum Bucket serves chum-based foods, all of which are shown to taste terrible. In "Plankton's Regular," the Krusty Krab adds chum to its menu in an attempt to steal Plankton's single regular customer.
The customer is disgusted by their chum, and says that it is the second foulest thing he has ever tasted. This implies that chum is inherently disgusting, and cannot be made to taste good, although this is shown to be false in the episodes "Greasy Buffoons" and "Chum Fricassee." In the former, Plankton uses excess grease from the Krusty Krab's grease trap that was dumped near the Chum Bucket, to make his sales skyrocket, a goal which he succeeded in. However, his business is lost when the health inspector comes to investigate both the Krusty Krab and the Chum Bucket due to the grease trend.
It should be noted that in real life, chum is made of grounded-up fish parts that are inedible to humans, such as heads and guts, and is used as bait for sharks and similar creatures. This would mean that Plankton is killing fish, or at least collecting dead fish, and serving their remains to other fish, thus promoting cannibalism. This has only ever been touched upon in "The Krabby Kronicle," where SpongeBob prints a story claiming this. However, SpongeBob was being tricked by Mr. Krabs into printing absurd lies to appeal to customers, so this story was supposedly false. As mentioned in the episode "Spongicus," Plankton named off many odd things that make up his chum, and fish parts were not on the list.
Plankton has no regular employees, and runs the Chum Bucket entirely by himself, although he spends most of his time in his laboratory, due to having no customers. Plankton runs the restaurant only when he has potential customers. In addition, when he does have customers, he also uses artificially intelligent machines to cover the other positions.
It has been stated on multiple occasions that Plankton has never had a single customer. However, he has had success on several occasions, albeit for a short amount of time, particularly in the more recent episodes.
In The SpongeBob SquarePants Movie, Plankton had exhausted all of his diabolical plans, and was forced to resort to "Plan Z." As per its instructions, he stole the crown of King Neptune and sold it to Shell City, framing Mr. Krabs for the deed. With Krabs cryogenically frozen by Neptune, Plankton took the Krabby Patty secret formula and became highly successful from producing the burgers. Using mind-controlling bucket helmets, Plankton turned Bikini Bottom into a Stalin-like dystopia named Planktopolis, where everyone serves the minuscule menace. However, he was suddenly stopped by SpongeBob and Patrick, who destroyed the mind-controlling helmets, thus freeing the Bikini Bottomites, and returned Neptune's crown to its rightful owner. Plankton was left crippled from being stepped on by the townsfolk, who incarcerated him in the Institution for the Criminally Tiny.
Deception: Plankton has engaged in many unusual acts of kindness in the name of personal gain:
In "F.U.N.," Plankton pretends to be in a friendship with SpongeBob so as to obtain the formula from him.
In "Walking Small," Plankton deceives SpongeBob in believing that berating the beach attendants is done with kindness so he can make an unauthorized development to the property.
In The SpongeBob SquarePants Movie, Plankton acts disloyal to the king of Bikini Bottom by stealing his crown and eventually enslaving him with a mind control helmet. Additionally, he betrays the town of Bikini Bottom by renaming it to suit himself and making the city revolve purely around him.
Taking advantage of others: When Plankton enslaves Bikini Bottom in the first movie, he forces Gary to pull a several-ton block of stone. In "Walking Small," Plankton uses SpongeBob for unauthorized land development and manipulates him into following his orders, deceiving him in believing that they are acts of kindness.
The need for power: As well as wanting the Krabby Patty secret formula, Plankton had a personality based on wanting power over others.
In the video game Battle for Bikini Bottom, Plankton creates a spade of robots to vandalize and threaten Bikini Bottom to increase his possibility of stealing the formula.
In The SpongeBob SquarePants Movie, Plankton brainwashes the citizens into worship him; once SpongeBob frees all the citizens from the bucket helmets, the police discover his motive and deal with him.
In "Chum Caverns," Plankton enslaves a group of cave dwellers.
In "Jailbreak!" his chum is used by criminals as knockout gas and disguises, both for robbery.
In The SpongeBob SquarePants Movie, to prevent SpongeBob and Patrick from making it to Shell City, Plankton hires an agent by the name of Dennis to deal with the duo. Dennis is eventually hit by a boat.
In "Spongicus," Plankton arranges for Patrick to be eaten by a group of lions for the entertainment of others.
Getting what he wants no matter what: Plankton breaks into SpongeBob's house in "Plankton!" to enter his brain. He also invades Patrick's house in "Salsa Imbecilicus" to get a copy of his DNA.
Plankton escapes from jail in the episodes "Krabby Road" and "Jailbreak!"
In The SpongeBob SquarePants Movie, he finally gets a hold on the Krabby Patty formula.
Disturbing the peace: In "Sweet and Sour Squid," Plankton is arrested for playing clarinet that caused a disturbance throughout town, even though it is Squidward's fault.
Plankton is married to Karen, a waterproof supercomputer. Plankton built Karen himself out of a calculator and a mass of wires. She was originally a simple security system, as revealed in "Friend or Foe," but has become more advanced as Plankton upgrades her and his other inventions with new technology. Karen lives in the Chum Bucket laboratory and is the source of his evil plans to steal the Krabby Patty formula. The two have a son named Chip Plankton II who Plankton initially wanted nothing to do with but changed his mind after he saves him and he brags on him. Chip later grows up and serves as an ATM machine at the bank. Through an experiment, he is one of half-father of PlanKrab.
Being a microscopic organism, Plankton's biological family numbers are in the thousands. In "Plankton's Army," he recruits his entire family to help him steal the formula, thinking them all to be criminal masterminds like himself. However, much to his surprise, his family consists entirely of uneducated, red-necked hillbillies. These relatives include, but are not limited to: Clem, Zeke, Rufus, Jeke, Billy Bob, Billy Jim, Billy Billy Bo Billy Banana Fana Fo Filly Plankton, Doug Plankton, Enis, and Rainchild, whom are the only ones named.
Plankton and Mr. Krabs have a long history of a feud over the Krusty Krab. When they were kids they were friends until they had problems over who invented the secret formula as shown in the episode "Friend or Foe." Plankton often makes schemes to steal them but always ends up losing. Though they are enemies, Plankton and Mr. Krabs work together occasionally, most noticeably in "Best Frenemies," and "The Other Patty."
Karen and Plankton are a married couple. "Friend or Foe" reveals that they started dating while Plankton was still in school. As stated in "Girls' Night Out," they have been married for 26 years. Because they have been married so long, they are prone to bickering and starting fights. Karen is the usual source of Plankton's evil plans to steal the formula, but he often takes credit for her ideas.
Plankton and Squidward barely interact due to the fact Squidward doesn't care about the secret formula. When they do interact they don't like each other, though Plankton pretended to like him in the episode, "Sweet and Sour Squid."
Plankton and Patrick have interacted a few times, but Plankton often dislikes Patrick due to his stupidity. In the episode "Chum Bucket Supreme" however, Patrick gives Plankton a slogan that makes his place boom in popularity, so he treats Patrick well in that episode.
Plankton is afraid of Pearl. This is shown in the controversial episode "One Coarse Meal." However, in other episodes like "The Algae's Always Greener" he doesn't mind her, he just finds her annoying and nagging.
Plankton and Gary have a strong dislike for each other. Plankton pretends to be him in "Shellback Shenanigans." And in the episode "Pineapple Invasion," Plankton and Gary fight because Plankton tries to get the formula.
Like Squidward and SpongeBob, Plankton has also had a large number of near-death experiences in the series, and this becomes a running gag in the series. He is usually stepped and trodden on by people because of his size, and when he angers Karen she will zap him. He is also sometimes launched back into the Chum Bucket by Mr. Krabs when caught stealing the Krabby Patty secret formula. He is also hurt when his schemes backfire.
In SuperSponge, Plankton has a brief cameo, in which he yells at SpongeBob for stealing his time machine: "Blast you, SpongeBrat! You have found my secret time machine! Well, whatever you do, make sure you fill it up before you bring it back!"
In Battle for Bikini Bottom, Plankton devises a plan to take over Bikini Bottom and obtain the Krabby Patty formula by creating a robot machine called the Duplicatotron 3000. However, the robots do not obey him and begin mindlessly rampaging across Bikini Bottom because he forgot to set the switch from "Do Not Obey" to "Obey." Plankton convinces SpongeBob and friends to defeat the robots so that he can have enough time to fix the main Duplicatotron 3000 to take over the city by controlling the robot army.
In Lights, Camera, Pants!, Plankton appears as a playable character. In the game, he uses a robotic suit to act in the auditions. In addition, the "Captive" role, as well as the auditions for it, take place in the Chum Bucket.
In Nicktoons Unite!, Plankton is one of the four main villains of the game. He uses a crab mech to fight the heroes.
In Creature from the Krusty Krab, Plankton is one of three playable characters, as well as the final boss of the game. He enlarges a Krabby Patty to massive size, only to have it come to life and begin chasing him. Eventually, Plankton enlarges himself to gigantic size, and begins rampaging across the city in pursuit of the giant patty. He is eventually defeated and reverted to his original size by Patrick, who was playing as his alter-ego Starfishman.
Plankton has been shown to be highly intelligent and has invented numerous things throughout the series:
Karen: Plankton built Karen himself from a calculator and a mass of wires. She is a waterproof supercomputer with many talents, like analyzing anything. She can also create holographic food. Apparently she was Mr. Krabs' and Plankton's security system when they were co-running their first restaurant.
In the episode "One Coarse Meal," it is revealed that Plankton is afraid of whales, enough to go insane and attempt to commit suicide. The reason is that a pod of whales once ate his ancestors. He also fears his plans failing many times.
""Well, we like to take long walks on the beach, and, uh, we like to talk about our dreams. Our dreams of conquering all of Bikini Bottom and become supreme overlords of all creation and to trample our enemies beneath our feet."" —"Single Cell Anniversary"
""Thanks to your brilliant AIM, honey, now I'm stuck inside that fool of a fry-cook!"" —"The Inside Job"
""There's nothing a little duct tape and surgical tubing can't accomplish. Ow! I knew I should've used pipe cleaners. Hello, I'm your long-lost relative. Can I have the secret formula?"" —"Sweet and Sour Squid"
Although absent from the final product, Plankton appears in a deleted scene from "Sandy's Rocket," where he is captured by SpongeBob and Patrick when trying to sneak into the Krusty Krab. In a chapter book adaptation of the episode, which was based on the original script, this scene is used. 
There is a running gag in many episodes he appears in where he is squished by a person.
Another running gag is him being projected through the air back to the Chum Bucket, typically by Mr. Krabs, often screaming a sentence or a prolonged word meanwhile.
It is revealed in "Single Cell Anniversary" that Plankton is not actually green; his outfit is. Karen removes the green and the viewer sees an underwear-clad pink Plankton embarrassed by his nakedness. He also has a rack with identical green suits.
Plankton and Mr. Krabs were born exactly on the same date, as revealed in "Friend or Foe."
Plankton is college educated, which has become a running gag throughout the series, as he would usually shout "I went to college!"
Stephen Hillenburg says that chronologically, The SpongeBob SquarePants Movie is the series' end and thus takes place after every episode. However, in the movie, Plankton says he's never had a customer, but in "Chum Caverns" and "Chum Bucket Supreme," he has one at least, and usually more than his only competition, the Krusty Krab.
Out of his whole family, he is the smartest, as the rest of his family is shown to be clueless and unintelligent.
Plankton's family is made up of hillbillies, and he appears to have forgotten their stereotype after all these years, as in their episode, he believes they are "criminal masterminds" but when he sees them, he is shocked and says "I've been away from home longer than I thought."
According to "Truth or Square," Plankton has made 1,003 failed attempts to steal the formula.
At two inches tall, Plankton is actually gigantic for his kind. The creators did this to make viewers see him on the screen.
Plankton actually should be half the size of SpongeBob, as the latter is four inches tall.
In the episode "Sweet and Sour Squid," Plankton is revealed to have ears.
Plankton finds Mr. Krabs' life to be worse than his life when he experiences it in "The Algae's Always Greener" and "Spy Buddies." In the former episode, he found numerous reasons that make Mr. Krabs' life difficult:
SpongeBob is always asking him for a Weekly Performance Review on Tuesday and throws a minor temper tantrum if Plankton refuses. He also suffers nervous breakdowns when found guilty of minor errors, such as using too much sauce on a Krabby Patty or giving a customer a large drink when a medium-sized one was ordered.
Squidward complains to Plankton that he does not want to work with SpongeBob. Later, when Plankton puts him in charge of fry cooking, a customer gets angry because the Krabby Patty that Squidward cooked is unappetizing.
He learns about teamwork in The SpongeBob Movie: Sponge Out of Water.
The analysis of what is inside Plankton changes. Karen's screen in "Plankton!" shows he is 1% evil and 99% hot gas. In "Plankton Gets the Boot," he is 30% mean guy, 20% spiteful monster, and 50% evil butt-head, total of 100% big jerk.
Plankton's phone number is 1-800-328-2486 or 1-800-EAT-CHUM according to the episode "Gary's New Toy."
Plankton once had an official SpongeBob Twitter account, at @SheldonPlankton, but it was suspended. It can still be seen at an archive.
Tweets mentioning the @SheldonPlankton account from Karen's and Mr. Krabs' Twitter accounts are still up. Examples here and here.
According to the episode "Pineapple Invasion," Plankton does not like the taste of pineapple.
Plankton's sizes drastically vary in the episodes: in some episodes he is the size of a jelly bean (e.g. F.U.N.); in others he is the size of a Krabby Patty (e.g. For Here or to Go), and in others, it is difficult to see him without a magnifying glass (e.g. Plankton!).
Sometimes, his size even changes within an episode. The only consistency is his being small enough to fit in the palm of one's hand.