SpongeBob SquarePants is the title character of the series of the same title. He has appeared in every episode of the series and obviously has the most appearances of any other character.
Although SpongeBob has appeared in every full-length episode of the series, he does not appear from several shorts. These include the What If SpongeBob Was Gone? shorts, Friend or Foe shorts, and the Merry Nickmas shorts Patrick the Snowman, Plankton's Holiday Hits, and A Chuckie Finster Christmas, Channukah, Kwaanza, Winter Solstice. Although, he has appeared in many other shorts.
He played a major role in all episodes, except Rise And Shine, Ugh, Plankton's Army, New Leaf, Best Frenemies, SB-129, Enemy In-Law, The Algae's Always Greener, Mermaid Man and Barnacle Boy, Imitation Krabs, Tea at the Treedome, Squidward, the Unfriendly Ghost, etc.
The Primary Universe SpongeBob does not appear in the episode The Algae's Always Greener. Instead, SpongeBob's alter-ego SpongeBob SquarePants (Mr. Plankton Universe) appeared.
His only non-speaking appearance is in Reef Blower.
Episodes in which he does not play a major role
Print in bold means character played big role
Secondary role = s
Minor role = m
Cameo = c