Ghosts

A garden-variety ghost is a deceased soul that clings to the mortal realm due to some obsession (a tradgedy they can't move past, a task they feel an oppressive need to complete, an unhealthy level of interest with some hobby, etc). The ghost's ability to manipulate the physical realm - it's strength - is proportional to two things: The strength of the deceased's soul (like all super powers - and like all super powers, it can continue to grow with time and exercise), and the strength of the obsession. Among other things, this generally means that a ghost who is coherent enough to carry on a conversation is usually going to be barely able to make a candle flicker, while a ghost that can actually hurt someone is going to be utterly bonkers.

There are a few traits common to all ghosts:

First, they are seeen and heard with the soul, not the eyes and ears, so only creatures with souls can experience ghosts directly. They don't show up in mirrors, they can't be photographed (although a sufficiently strong ghost that understands the camera in use can manipulate the resulting image), minions and undead can't see them, and so on.

Second, ghosts are universally transparent and do not cast shadows for the same reason: They don't block light (a particularly strong ghosts can cause enough 'mental dissonance' that most people ignore whst they can see through the ghost because the soul-presence of the ghost is strong enough to overwhelm physical perceptions... but the light still gets through).

Third, their appearance is based on their self-image, which isn't easily changed. As death is often a traumatic affair, it is common for the fatal blow to be reflected in the ghost's form: Gaping wounds, profuse bleeding, broken limbs, and so on. Additionally, the ghostly image is generally either idealized (if they liked themselves in life) or twisted (if they focused on their flaws) view of who they were while alive.

Fourth: They're already dead, so they can't be killed.

Also: Ghosts, being souls, are not technically undead.