Who is Jesse from Ninth Key and what is their importance?

Jesse is the handsome one-hundred-fifty-year-old ghost, who lives in Susannah's bedroom. He protects Susannah until the last chapter, when she hatches an escape plan of her own. Over the course of the novel, Susannah is torn between helping Jesse get to the other side and having him stay with her. She remains selfish in a childlike way when it comes to keeping Jesse in her life. In essence, Jesse and Susannah stand in the way of each other crossing over to where they need to be. Jesse also plays a role in helping to guide Susannah in her novice mediation efforts; most likely, Susannah will not be able to help Jesse get to the other side until she has grown up some more.