Karen Plankton is the main computer system of Sheldon Plankton's laboratory, as well as his sidekick, best friend, and wife. She usually comes up with plans to steal the Krabby Patty secret formula, which Plankton tends to claim as his own ideas. Karen was introduced in the episode "Plankton!" along with her husband. She is a main character and one of the two main antagonists. As revealed in the "Friend or Foe?" special, Karen was just a security system when she started dating Sheldon. Karen has a mother named EMILP, who is seen in "Single Cell Anniversary" as a computer program. Her father has never been seen in the series, but appears in an issue of Nickelodeon Magazine.
She has a Chicago accent, in contrast to Sandy the Squirrel's Southern accent, and Plankton's Lithuanian-Estonian one.
In Karen's first appearance, Plankton planned to use her analyzer program to decipher the Krabby Patty formula. In the episode: Fear of a Krabby Patty, she had eyes, a nose, and mouth telling Plankton "I heard this joke before". Karen and Plankton argue amongst themselves for Plankton had installed a nagging software to Karen. Karen finds Plankton's first name (Sheldon) funny, as we see in Plankton's Army.
In the movie, Karen plays an important role. She informs Plankton of "Plan Z," setting into motion the rest of the film's events. The movie also marks the first time Karen is seen in an updated form; Karen is smaller and now has wheels. Plankton also installed a button into Karen that would help him succeed in Plan Z and take over Bikini Bottom.
She has a green wave on her screen to tell in which frequency she is speaking at. She has many buttons below the screen. Clem and the rest of Plankton's Cousins think that she is pretty. She has a built in analyzer, which can tell the ingredients in an item. In the majority of her appearances, Karen manifests as a large monitor in Plankton's lab or as her mobile, wheeled form. She is also installed into most of Plankton's inventions, such as Robot Krabs, Plankton's seashell submarine (as seen in It Came From Goo Lagoon), and the Chum Bucket's fist itself (as seen in 20,000 Patties Under the Sea). In the episode Goo Goo Gas, she is shown as a baby calculator. As a baby, the only word she knows is "Hello."
- "Seaweed: 50% Sea, 50% Weed. (Later towards the end of the episode) Plankton: 1% Evil, 99% Hot Gas." (Plankton!)
- "I heard this joke before!" (Fear of a Krabby Patty)
- "Just blink." (Komputer Overload)
- "Aw, I think you're just being overlydramatic." Plankton's Army)
- "So, typical day of failure, I see, huh darling?" (The Algae's Always Greener)
- "Don't you dare!" (Single Cell Anniversary)
- "I never agreed." (The SpongeBob SquarePants Movie)
- "I suppose it would, if I had one."
- "SHELDON!! HA HA HA!"
- " The self-destruct coin slot has been activated. Ten seconds till detonation." (Imitation Krabs)
- " Loading E.M.I.L.P."
- "Here's your stir fry, little man!" (Karen 2.0)
 Episode Appearances
- Plankton! (debut) (Season 1)
- F.U.N. (Season 1)
- Imitation Krabs (voice only) (Season 2)
- Welcome to the Chum Bucket (Season 2)
- The Algae's Always Greener (Season 3)
- Plankton's Army (Season 3)
- The SpongeBob SquarePants Movie
- Fear of a Krabby Patty (Season 4)
- Krabs vs. Plankton (Season 4)
- Dunces and Dragons (Medieval Karen) (Season 4)
- Enemy In-Law (Season 4)
- Best Frenemies (Season 4)
- Friend or Foe? (Season 5)
- Krabs a la Mode (Season 5)
- Bucket Sweet Bucket (Season 5)
- Goo Goo Gas (Season 5)
- Mermaidman vs SpongeBob (Season 5)
- Banned in Bikini Bottom (Season 5)
- Krabby Road (Season 6)
- Spongicus (Season 6)
- Patty Caper (Season 6)
- Plankton's Regular (Season 6)
- The Krabby Kronicle (Season 6)
- Komputer Overload (Season 6)
- Single Cell Anniversary (Season 6)
- Truth or Square (Season 6)
- Chum Caverns (Season 6)
- Clash of the Triton (Season 6)
- Someone's in the Kitchen with Sandy (Season 7)
- The Inside Job (Season 7)
- SpongeBob's Last Stand (Season 7)
- One Coarse Meal (Season 7)
- Buried in Time (Season 7)
- Shellback Shenanigans (Season 7)
- Perfect Chemistry (Season 7)
- Frozen Face-Off (Season 8)
- Sweet and Sour Squid (Season 8)
- A SquarePants Family Vacation (Season 8)
- Walking the Plankton (Season 8)
- Plankton's Good Eye (Season 8)
- Karen 2.0 (Season 8)
- It's A SpongeBob Christmas! (Season 8)
- Move It Or Lose It (Season 8)
- Evil Spatula (Season 9)
KarenPlanktonamorPlanktonEugene H.KrabsJack M. CrazyfishOld Man JenkinsSpongegarSquidwardPearlThe Drifter