The rainbow is a character who appears in the film The SpongeBob Movie: Sponge Out of Water and the book The SpongeBob Movie: Sponge Out of Water: The Junior Novelization.
He is a giant, sparkly, sentient rainbow produced by Plankton who lives in SpongeBob's brain. Like the other creatures inside his brain, he is a figment of SpongeBob's imagination. He contains all of the colors of the rainbow. His face has big eyes with eyelashes and rosy cheeks. He also has two purple arms.
Role in movie
When Plankton encounters the giant cute kitty in SpongeBob's brain, he gets sick and barfs out a big sparkly rainbow. The rainbow comes alive and calls Plankton "Daddy," which shocks Plankton and causes him to leap out of SpongeBob's brain due to there being too much sweetness.