J.D. is Steven and Jen's baby as seen in "Little Monster." J.D.'s vocalizations were provided by Jessica DiCicco.
Noodles originally disliked him because he was getting all the attention.
His owners even threw away Noodles' favorite toy- a stuffed lion -after J.D. spit up on it, which upset Noodles very much, but he warmed up to J.D. eventually and Steven ended up saving Noodles' stuffed lion from the trash and cleaned it. It's very likely J.D. will grow up into Noodles' perfect person, but he won't remember the dogs talking because babies have very small underdeveloped brains.