Who is Adam from Haunted: A Tale of the Mediator and what is their importance?

Adam is one of Susannah's closest friends in Carmel. He has his own car, so he Susannah and Cee Cee use it to get around to different places in the novel. Adam is the one who drives them to the mall to pick up the cat, Spike, who belongs to a ghost of a boy that appears to Susannah.