Will o' Wisps are floating balls of colored fire associated with ghosts.
Will o' Wisps are commonly associated with ghosts in purgatory, and one always trails by the side of its respective ghost at all times. The color of the wisp indicates what type of ghost they are, and this additionally coordinates with their eye color (with the exception of Sternenglanz). As this is ghostly fire, they require no fuel supply or oxygen to keep them lit, they cannot be extinguished by external means such as water, and they are unable to burn objects unless the respective ghosts wills it (as shown when Kleinod occasionally uses his to light his cigarettes when he readily lacks a traditional lighter). They are not independent spirits, but separate parts of the ghost's soul.
Other than aesthetic, will o' wisps reflect the health of its respective ghost; if a ghost's physical health wanes, the flame gets smaller, and in instances where the ghost or priest is dying, it progressively shrinks until it is completely extinguished, coinciding with the death of its owner. Will o' Wisps can be divided into separate pieces, willingly or otherwise, and they can be used to create independent, quasi-corporeal human alter egos or personas (such as when Kleinod and Weltraum create Yuki and Saito Amano to inhabit Kotori's limbus realm as a way of monitoring her while he and continue reaping souls), or, more commonly, they can be consumed by other ghosts to gain their powers by a small amount. They are also used frequently in soul reaping for viewing a deceased person's life events and method of death, and spirits being reaped and interviewed are shaped like Will o' Wisps in their idle form, with a miniature version of the respective soul seen within the fire.
- From the end of episode four onward until the end of the series, Kleinod and Weltraum's Will o' Wisps are slightly smaller than before, signifying that parts of their souls have been dismembered (coinciding with Weltraum's shorter hair and Kleinod's thinner appearance).
- Pluto and Persephone lack Will o' Wisps of their own since they are much more hardy and powerful than normal ghosts, owing to them being humanity's first ghosts.
- Although alternate human identities can be created from a ghost's Will o' Wisp, they do not serve as replacements for priests, who are ghosts that regained their human bodies through Pluto's will.