Incidental 21 is a quiet light orange fish who makes cameo appearances throughout the series.
He is a light orange fish with big orange eyes and has brown fins and lips. He also wears a cyan colored shirt. He has never spoken in the show but his mouth can be seen moving on rarest occasions.
He is usually seen as a customer at the Krusty Krab. He makes his debut in "Ripped Pants" as one of the many fish surrounding Frank while he is holding SpongeBob who has believed to be drowned. In "Christmas Who" he sings "The Very First Christmas" along with a bunch of citizens. He also appears in the episode "Bulletin Board" several times as one of the many customers participating in the Krusty Krab bulletin board. He can clearly be seen in the beginning of "The Hankering" sipping a soda at the Krusty Krab. He has appeared as a quiet background character in all of his appearances so far.
He resembles a realistic looking fish while most of the fish in the series have a more cartoonish look.