Vernon is likely a character within the Psychonauts universe. In the context of the Vernon Ghost Story, he's probably the central figure who has some sort of encounter or relationship with the ghosts.
Vernon could be anyone. Maybe he's an old man in the neighborhood who loves to share spooky tales from his youth. Or perhaps he's a young person who had a strange encounter and turned it into a ghost story. Without more context, it's hard to say exactly who he is.
Davina could be an ally too. She has a good heart and her own powers as a witch. She might be sympathetic to Hayley's situation and use her magic to create protective spells around Hayley and the baby. Also, she could act as a mediator between Hayley and the other factions in the city, given her relationships with different groups.
Petunia could be a great addition. Her relationship with both Harry and Vernon adds a lot of complexity. She could be the mediator or the one causing more trouble between them, depending on the plot.
One great Klaus and Hayley fanfic is 'A New Beginning'. It beautifully explores their complex relationship in a post - war scenario. The author does an amazing job of showing how they support each other through difficult times.
In some fanfiction, Uncle Vernon develops by becoming more accepting. For instance, he may start to see Harry not just as a freak but as a person with unique abilities that can be useful. He might begin to tolerate the wizarding world a bit more, especially if he realizes that Harry can use his magic to protect the family from certain threats.
Maybe Vernon is jealous of Harry's magical abilities. He might see Harry as a threat to his 'normal' family image. Also, Vernon has always been a bully at heart and Harry, being an easy target in his house, becomes the victim of his pent - up anger.
Love and sacrifice are also popular themes. Their love for one another may lead to them making sacrifices. This could be sacrificing their own safety or their own desires for the sake of the other.