In addition to the ten main characters, SpongeBob SquarePants has a large amount of background characters. They are identified by the artists and animators with individual model numbers. These model numbers correspond to when the characters were designed in production. The following is a list of background characters, sorted by their model numbers.
"Squeaky Boots," the second episode made for season 1 in production order (code #102), was the first produced episode to feature incidental characters. As a result, the first row of incidentals (incidentals 1-11) were all first designed for "Squeaky Boots." Despite this, Incidental 18 appears in that episode as well.
Some characters, such as Incidental 35 and Incidental 70, have been removed from the model sheets, mainly between the second and fourth seasons. Like most animated series, many SpongeBob episodes are worked on at the same time, leading to several incidentals with later numbers appearing in episodes before others with earlier numbers.
A crew made out of four pirates first appear in "Dying for Pie." The third pirate is the captain. An additional group of pirate Incidentals were added to this group for "Grandpappy the Pirate", those are edits of pre-existing character models with new pirate outfits.
These incidentals first appear in the "Friend or Foe" flashbacks, along with other regular incidentals reduced to the size of a child. The "K" very likely stands for kid. This is also the only incidental sheet where every member in it is known.
A total of five incidentals whose model numbers start with "M" (possibly standing for musician) appear in "Krusty Katering." They are recycled from other models, just recolored and dressed up with suits. Only three numbers are known: M2, M3, and M4.
Most background characters have two models: one for their daytime color palette and another for their nighttime color palette. For example, Fred's palettes are labeled "001-DAY" and "001-NIGHT."
According to Jay Lender, several of the original incidental characters were removed from the model sheets during the second season after their models were lost in the shuffle during revisions. As a result, many episodes featuring them in storyboards (such as "Christmas Who?" and "Mermaid Man and Barnacle Boy III") had them replaced with different character models.
The numbers for these characters are known from storyboards. Specifically, numbers 33, 35, 61, 62, 70, and 71 are known from the storyboards of "Christmas Who?."
It's been proven possible that if a removed incidental has its model retrieved it could return to the show and stay. For example, Incidentals 95-101 returned by "The Inmates of Summer" and Incidental 33 returned by "Patrick's Tantrum". Why the official model sheets haven't updated since then is unknown.
Fred, Scooter, Old Man Walker, and Sandals are currently the only incidental characters with official confirmed names by the SpongeBob crew. Other characters have been given various names throughout the show, but these are limited to single instances and do not carry over to other episodes.
The models for Incidentals 30 and 30A are redraws of the anchovies from "Help Wanted." Unlike the anchovies in "Help Wanted," these incidentals are solid blue and brown, and the brown anchovies have blue shirts instead of white ones.
In many episodes, duplicate background character fish can be seen in crowds.