Duke is the rare existence of a poltergeist dog in hell, as most animals don't stay in hell for eternity. Animals don't have the same morals and concept of guilt that humans do. Hell is created for those that feel they did something wrong and need to kept away to overcome their own eternal struggle. A mortal soul, will eventually get over it and default into peace, content, and joy. For a Poltergeist, you agreed to a deal that would keep you going on forever, with the powers the demon would grant you. Fused souls for eternity.

Duke's actual story isn't easy to figure out because he can't talk. Grunder figures is that he must've been owned by a human because he's always looking out for someone he'll never find. He's one of the really early Dog breeds, likely lived during the ice age. What caused Duke to accept a demonic alliance isn't sure, and maybe he never got laid.

If I have to break the forth wall, Duke was a hunting dog with an early man, and became so loyal that when up against foes, witnessed his master being taken away and became determined to tracked him down. While the arrows are the cause of his death, he traveled the land going after everyone he could, looking for his master. The blur between living and death doesn't register to him. He died and continued his quest in hell. He's been a pest and a friend to many other mortals in hell, and Duke is just the name Grunder gives him after they meet.

Duke doesn't get along with many people, but he and Demion DO NOT get a long. Demion insisted that there were no animals in hell but even he knows that regret and guilt can hit every creature in one way or another. He can take comfort knowing that plant poltergeists are even rarer, but nothing is impossible.