Plankton was first created by Stephen Hillenburg as a potential villain for the show. However, he later feared that Plankton's stories would get repetitive and predictable. During the production of season one, Hillenburg demoted Plankton from a main character to a minor one; he insisted that Plankton would work better as a one-time guest star.
However, Doug Lawrence loved the character. He summarized Plankton's origins in 2015, saying that he "was only supposed to be in one or two episodes, but I was a writer on the show and I really liked this character." Lawrence wrote episode ideas for Plankton and passed them to Hillenburg. From then on, Plankton began to appear more often. Lawrence's most notable contribution was giving a personality to Plankton's "waterproof computer," named Karen after Stephen Hillenburg's wife. He decided that Plankton and Karen should be an inseparable duo, with Karen as the brains of the operation, always supplying Plankton with evil plans.
Since Plankton's "reinvention" by Lawrence, he and Karen have appeared in many episodes. They were both officially promoted to main characters in the credits of the 2004 film and have been part of the main cast ever since. Plankton has also appeared in various video games.