Help me with subplots for my story?

I'm writing a story called Annie's Ghost. It's about a student named Annie Hatter who attends a regular boarding school in our regular, non-fantasy world. But when she moves into her study, she finds a ghost named Cadmus living there. He's trapped there, unable to leave the study. Annie is for some reason the only one who can see him, and so she decides to befriend him - but it soon passes beyond friendship. When you can't touch the one you begin to love, and they're the only one who can see you, what's a ghost to do?