In Fazbear Entertainment
Balloon Boy is an easter egg in Fazbear Entertainment, and appears randomly after lowering the cameras.
He does not kill the player, but he laughs and jams the doors and lights as if Bonnie or Chica have gotten in.
To get rid of Balloon Boy, pull up your camera, similar to Golden Freddy.
In Fazbear Entertainment 2
Balloon Boy is a non-playable character in Fazbear Entertainment 2.
He starts at the Game Area, then moves through the Main Hall (not appearing on the camera) and the Hallway to Party Room 1 (again, undetected) and the Left Air Vent. After appearing in the left vent light, he will appear in your office and disable use of the vent lights and flashlight.
After you put on the Freddy Fazbear Mask, he returns to the Game Area. The only way to get rid of Balloon Boy once he is in your office is to wait until 6:00 AM.
In Fazbear Entertainment 3
Although unconfirmed, Balloon Boy is likely to reappear as a Phantom in Fazbear Entertainment 3. Whether he will be a playable character is unknown.