Creation and conception
Stephen Hillenburg originally conceived early versions of the SpongeBob SquarePants characters in 1984, while he was teaching and studying marine biology at what is now the Orange County Marine Institute in Dana Point, California. During this period, Hillenburg became fascinated with animation, and wrote a comic book titled The Intertidal Zone starring various anthropomorphic forms of sea creatures, many of which would evolve into SpongeBob SquarePants characters, including "Bob the Sponge", who was the co-host of the comic and resembled an actual sea sponge as opposed to SpongeBob. In 1987, Hillenburg left the institute to pursue his dream of becoming an animator.
Patrick, Mr. Krabs, Pearl, and Squidward were the first other characters Hillenburg created for the show. Many of their characteristics were based on Hillenburg's experiences during his time at the Ocean Institute or inspired by the traits of their species. Patrick's personality embodies the nature of the starfish; according to Hillenburg, they look "dumb and slow" but are "very active and aggressive" in reality, like Patrick. Hillenburg drew inspiration from his former manager at a seafood restaurant while creating Mr. Krabs. According to him, this manager was redheaded, muscular, and a former army cook; these traits were all adapted into Krabs' character. His decision to design Pearl was influenced by his regular supervision of whale watches at the Ocean Institute, as well as by a cetacean skeleton at the institute. He drew Pearl with an oversized, almost geometric head as a reference to sperm whales having the largest brain size of any extant animal on Earth. He designed Squidward as an octopus because of the species' bulbous mantle; the octopus, he said, has "such a large bulbous head and Squidward thinks he's an intellectual, so of course he's gonna have [one]." Hillenburg drew Squidward with six tentacles because "it was really just simpler for animation to draw him with six legs instead of eight".
Several additions were made to the series' main cast before and after Hillenburg pitched the series to Nickelodeon; in his series bible, he added Sandy Cheeks the squirrel as a new friend of SpongeBob. Plankton and Karen were included in his bible but were not meant to make regular appearances; Plankton's voice actor Mr. Lawrence said 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". Following his first voice recording, Lawrence drafted some of his own ideas, hoping to "prove Plankton could survive as more than a one-note character." From then on, Plankton and Karen's roles in the series grew as Lawrence wrote ideas to give them more personality; notably, he decided to write Karen as Plankton's wife, rather than just his computer as was originally intended. They were both officially promoted to main cast members in the credits of the 2004 theatrical film, in which they play central roles.
Hillenburg added Mrs. Puff in response to a request by Nickelodeon that SpongeBob attend school. Nickelodeon executives initially wanted to make SpongeBob a child since their most successful cartoons at the time focused on young, school-age characters. Hillenburg stated that the network wanted SpongeBob to be like "Arnold [from Hey Arnold!] under the sea," but he told them, "No, that's not the show." As a compromise, he decided to put SpongeBob in a boat-driving school, allowing him to keep writing SpongeBob as an adult while also using the school as a main plot element. Showrunner Vincent Waller suggested that if Nickelodeon had creative control over SpongeBob, almost every episode would take place at Mrs. Puff's school, rather than at a variety of locations. The choice to make Mrs. Puff a pufferfish, who inflates into a ball when SpongeBob crashes, was made to evoke the appearance of car airbags.