JJ, originally from Five Nights at Freddy's 2, was an easter egg found underneath your desk. JJ is a Balloon Boy recolor with purple and blue stripes. She has no use in the game, and can't jumpscare you.
In FNaF World, JJ, just like most characters, can be playable once defeated when battling.
In Freddy Fazbear's Pizzeria Simulator, She is in UNC. She will come in the vent and you have to close the ventalation to stop her from entering. If you choose to let her in,she will most likely but peeking at you and take control over your doors, until she gets bored and leaves.