Plank is the imaginary friend of Jonny 2x4. He is a plank of wood with eyes and a mouth painted on. Although Plank is claimed to be a living being, he is never heard or seen moving on screen, but Jonny seems to be able to understand what he is saying; however, this is likely just Jonny's imagination.

It is massively questioned whether Plank really is alive or not due to the various implications of Plank moving, such as the scene in "Gimme, Gimme Never Ed." In the episode, Eddy seems to dislike him for hurting him in various ways. Plank is somehow enrolled in Peach Creek Jr. High and plays on the football team. In "Too Smart for His Own Ed," he participates in the annual school spelling bee (although he gets kicked out in the first round because nobody besides Jonny hears him speak).

Plank is almost always seen with Jonny. The two are near-inseparable and do everything together. For example, when Jonny became Captain Melonhead in the episode "Robbin' Ed," Plank became Splinter the Wonderwood, Jonny's sidekick. Plank is implied by Jonny to be a bit of a prankster and a wiseguy, and is often blamed for causing his troublesome predicaments. Jonny is so engrossed in the world of his best friend that he even created parents and friends for Plank out of other pieces of wood laying around.