There is a book called "Marcelline Diary".
It explain most of her backstory. Her mother was involved with weird shit when she was younger, and Hunson actualy loved both of them but needed to go back to Nightsphere.
She manages to summon him again with the help of some werewolfs that were taking care of her after Simon left. The first thing that Hunson did was eat the souls of those were wolfs, which obviously freaked her out.
Them they spent some time together wandering across the post apocaliptic world, and he was always super nice with Mareline, but he also killed everything around him, which made her rationalize that he was more of a force of nature.
Once they found the werewolf camp, where the last ones were fleeing afraid of vampires, she made her dad promess that he would help them. He them ate the souls of all living werewolfs in the same day as the events of the "fry song" - where she basicaly snapped and sent him back to Nightsphere.