"You will know you are the Chosen One when you sacrifice the life you love to save the family you love."
―Ghost of the Grand High Vampire

A ghost is the spirit of a dead person that can be seen by the living.

Only one ghost has appeared in the series. The ghost of the Grand High Vampire appeared before Vladimir Dracula to tell him that he was the Chosen One. Vlad was the only person to see the ghost. Ingrid and Will were not able to see him.

Later in the series, Ryan appeared before Erin to tell her that he'd been murdered by Count Dracula. Murderer in the Midst Vlad told Erin that vampires couldn't become ghosts. The apparition later turned out to be Elizabeta Vaccaria, a shapeshifter who took Ryan's form. Why the Grand High Vampire became a ghost is unknown, but it seems he was an exception.

The fact that vampires cannot turn into ghosts might suggest that humans can.