Many ghosts introduced in the first season appear to be the disembodied spirits of dead humans or other Earth creatures. Maddie once defined them as an "odd manifestation of ectoplasmic energy and post-human consciousness." This was supported by several ghosts' backstories, such as the Lunch Lady Ghost, Desiree, and Sidney Poindexter, which detail their lives as humans before becoming ghosts.
However, ghosts introduced in the second season onward became less human and more otherworldly in backstory and appearance. Creator Butch Hartman eventually stated that ghosts were in fact otherworldly monsters and creatures that came from the Ghost Zone, with no connection to human spirits.
Ghosts' bodies typically take on an anthropomorphic figure. Ghosts typically have pale blue or green skin, and emit a distinct white aura around their bodies.
Some ghosts appear very similar to humans in appearance and behavior (Ember McLain, Youngblood, Walker). Some only vaguely resemble humans (Undergrowth, Vortex, Nocturn). Most are not humanoid at all, taking the form of shapeless blobs (Skulker, Bertrand), anthropomorphic animals (Wulf, the Vulture Ghosts), or monsters (the Ectopuses).
Ghosts' names have many different origins. Some ghosts retain their names from their previous life (Sidney Poindexter). Some have common human names or nicknames (Walker, Johnny, Kitty). Others choose their own names (Youngblood). Some are named after their powers or characteristics (Ember, Fright Knight, Undergrowth). Some ghosts' names are merely literal descriptions of what the ghosts are or do (Box Ghost, Lunch Lady Ghost).
Many ghosts have strong obsessions, which often lead them to their evil behaviors. For example, the Lunch Lady Ghost is excessively conservative about the cafeteria menu, Spectra is obsessed with her appearance, Walker is extremely strict with his rules, and Klemper has a compulsion to make friends. Ghosts' powers are usually related to these obsessions.
- Main article: Ghost Powers
All ghosts possess supernatural powers such as the ability to walk through walls, disappear, and fly. Ghost bodies are unaffected by gravity, so many ghosts have some degree of superhuman strength. Many ghosts possess other powers such as possession and ghost rays.
- Main article: Half-Ghost
A half-ghost is a human whose DNA is infused with ectoplasm. These humans possess many of the powers and abilities that ghosts do. Danny Phantom, the protagonist, is well known to be a half-ghost.