JJ is one of the three hobos that Finn and Jake confront in the Desert of Doom in "The Other Tarts." He and his compatriots Grimby and Taddle steal some of the (poisoned) royal tarts from Finn and Jake. JJ uses his rockets to escape from Finn and Jake and flies off with Grimby and Taddle into the desert.
JJ appears to be a sentient video game machine, much like BMO. Just as BMO has the letters "BMO" printed on the side of him, JJ has the letters "JJ." JJ's face has three eyes and he has a small crack next to his left leg.