Who is Ghosts - Capt. Alving from Four Major Plays and what is their importance?

Captain Alving never physically appears in the play, but his presence is felt throughout. The Captain is a constant topic of conversation, and each of the characters owes his or her state of being in the present to Captain Alving's essentially dissolute and immoral nature and his past behavior. Alving embodies the play's central theme...... that the past is inescapable.