Jack and Ralph are antagonists of Homeward Bound II: Lost in San Francisco. They (namely Ralph) drive a blood-red van, which they use to catch dogs and take them to the lab to be used in experiments. Jack is the dominant personality of the two. He smokes cigars and carelessly discards them.
Their first two appearances in the film are brief - Jack tries to steal a French poodle whose owner was in the shop buying something, resulting in him splitting his pants when he bends down, but her owner shows up, Jack chastises him that he should be "ashamed of himself" and that some scoundrel is in the area waiting to steal a dog and gives the French poodle a kiss much to her and owner's annoyance, and he has to make an excuse - while the dog urinates over his T-shirt in revenge. In the second but also brief appearance, Jack throws his cigar out the window of the van, and it sets a nearby house on fire. This also led to Shadow and Sassy rescuing Tucker (the son of the occupants' house, as well as the kid they met earlier and scared back inside by Chance) and his pet kitten, Tiger respectively (although the latter was unintentional, as Sassy was actually trying to arrive to dissuade Shadow from helping).
Their main appearance comes later in the film when they try to capture Chance. Jack distracts him with a cheeseburger, while Ralph sneaks around behind him with a catchpole and captures him. As they drive off, Jack gloats over their new captive; however, before they can get very far, Riley's gang, along with Shadow and Sassy, block the road at the docks. Jack realizes that the dogs are turning against them and orders Ralph to run them down. Ralph refuses on account of the money they could make from bringing all of them in. Jack and Ralph fight for control of the van, which veers out of control. When it nearly runs over Delilah's paw, Bando leaps into the van and drives Jack and Ralph out of it. While Riley and Sledge chase down Jack and Ralph, respectively, and drag them to the ground, Sassy frees Chance and the other captured dogs who then chase Jack and Ralph away, who climb up a fence and onto the other side (although not before Riley ends up ripping Jack's red underwear from outside his split trousers). Bando sets the blood-red van in reverse so it rolls backward off the harbor and into the sea, where it sinks. Jack and Ralph are not seen for the rest of the film and it is assumed that they are arrested by the police recognizing them as the culprits who set Tucker's house on fire, trying to steal a French poodle, capturing Chance, and their blood-red van is pulled out of the sea.