The viewer is an implied character in The Backyardigans.
The existence of the show's viewer is never confirmed, but the major characters often talk to the viewer (for example, saying "Hi!" to the screen) and occasionally directly refer to them. Whenever the characters use the word "you" and are not speaking to another shown character, they are speaking to the unseen viewer. In this instance, the character is breaking the fourth wall by recognizing a being from outside their universe.
The viewer's appearance is unknown, as their entire existence is not confirmed.
The viewer has never appeared in an episode of The Backyardigans, but is frequently spoken to and referred to. The viewer is first spoken to in "Pirate Treasure" and last spoken to in "The Tale of the Not-So-Nice Dragon".
One of the few instances in which the viewer is never talked to is in Nickelodeon's Mega Music Fest. It is unknown if the characters talk to the viewer or not in the live-action pilot "Me and My Friends" or in the animated pilot.