The Poltergeist King guards the Belmont family's sacred weapons until they are needed again. He leaves them throughout the Castle behind pillars. This character is only referenced in the English instruction manual for Castlevania III: Dracula's Curse and was never actually mentioned in the original Japanese manual, thus his inclusion is considered non-canonical.
The Poltergeist King has a prominent role in Episode 29 "Return to Castlevania" of Captain N: The Game Master. In it, Simon Belmont is about to be bestowed an honor in front of a large crowd of being a worthy hero like his great-grandfather Trevor by the Poltergeist King. However, Dracula takes his form and accuses his ancestor of taking the credit for his slaying of Dracula. In one scene, the Poltergeist King (or rather, Dracula in disguise), appears as a whirl-wind. This suggests that the Poltergeist King may have been responsible for the tornado that helped Simon Belmont in get around the Deborah Cliff in Castlevania II: Simon's Quest.