A half-ghost (or "halfa") is a human whose genetic structure is infused with ectoplasm. Half-ghosts have the ability to change between human and ghost forms at will, and possess the same supernatural powers that ghosts have.
Though the first half-ghost shown in the series is Danny Phantom himself, he was not the first half-ghost to come into existence. During Jack Fenton's and Vlad Masters' college days, Vlad became the world's first half-ghost when he was caught in the blast of Jack's prototype ghost portal. Twenty years later, Danny would go on to suffer a similar accident inside Fenton Works' completed ghost portal, becoming a half-ghost himself.
Half-ghosts have access to ghost powers such as intangibility, invisibility, and flight. Half-ghosts can use the majority of their powers while in human form, but in order to use them to their full potential they must change into ghost form, a transformation which Danny refers to as "Going Ghost."
Half-ghosts have immunity to many effects due to their dual nature. For example, they can pass through anti-ghost shields unharmed while in human form, and their ghost half makes them resistant to ghostly effects, such as Ember's hypnotic music, that would otherwise affect humans. However, like full ghosts, they are still vulnerable to ghost weaknesses such as ecto-ranium and Blood Blossoms.
- Not everyone can become a half-ghost. For instance, if Sam or Tucker had gotten caught in the middle the Fentons' malfunctioning ghost portal, it would have likely killed them.