Rex used to be a Ranger (like Ella). He showed her the ropes before he retired. He’s a widower and owns the property that Ella parks her 5th wheel on. A 5th wheel is a trailer. That’s where Ella lives while working as a ranger.
When Ella had to go away at the end of the Hellbeast King (book 1) she called her friend Rex to look after her beloved dog Duke.