Ghosts & Spirits

Among the less common threats to humans are ghosts and spirits. While ghosts are a fairly common occurrence (especially around Karasumori), they seem to rarely become threats, and are typically not treated as such unless they prove aggressive toward humans. Spirits are more elusive, especially from a Kekkaishi standpoint, but this is likely because spirits are the natural enemies of exorcists, just as Ayakashi are the expected enemies of Kekkaishi.

A ghost is the soul of a dead human or animal, still present in the real world. For the most part, they are partially transparent (though entirely invisible to those who are spiritually unaware), capable of flight to some degree, and able to pass through matter (and are likewise unable to touch most matter). A Kekkaishi's approach to handling ghosts appears to depend largely on personal preference: Yoshimori typically directs them to Yumeko Hananokoji, and tries to reason with them even if they prove threatening. Tokine ignores them and suggests that Yoshimori do the same, but despite repeatedly telling Yoshimori that it's best to destroy them, never actually seems to carry out this threat herself. Shigemori Sumimura appears to treat them as threats (at least, if they appear in his home). Tokine suggests that a ghost's presence in the living world destroys the natural balance, and that they could have horrible effects on people's bodies (though Kekkaishi have built up a tolerance to this), but she may have been exaggerating to stress the point that ghosts, like Ayakashi, have the potential to become dangerous, especially those near Karasumori for a prolonged period.

Spirits are considerably more powerful than ghosts (and may simply be vastly experienced ghosts who have become more powerful over time), and Kekkaishi seem ill-equipped to deal with them. Kiyoko's case suggests that being a wandering spirit is essentially a trapped state: they cannot pass on, become Ayakashi, or reincarnate, and lead largely empty existences. Evil spirits are dealt with either through sealing or exorcism.

Possible Paths of the Dead
There is some confusion (largely due to "ghost" and "spirit" being used interchangeably at times) surrounding the possible fates of the dead.

After Death, The Living Become
Ideally, Kekkaishi are either able to destroy threatening ghosts, or help them pass on before they can become a threat. A few ghosts (such as Gen-san or the Janitor Ghost), however, are stated to have been around Karasumori for quite some time after their deaths, yet display no signs of transforming, and are considered non-threatening (by Yoshimori, at least, but their continued presence indicates that no other Kekkaishi has not destroyed them, either). After his original death, Madarao was unable to pass on (indicating that he lingered on, likely as a ghost), and eventually became an Ayakashi. This also applied to Kouya, and most probably Hakubi. This implies that ghosts can become Ayakashi over time, which is supported by the typical Kekkaishi attitude toward ghosts (that they can become threats if left unattended), and the case of Masano Murakami. Kiyoko clearly recalls being alive, if only briefly, before her death. This implies that at least some spirits were once alive and/or ghosts. In the anime, Kaguro dies as a human and becomes an Ayakashi instantly. While this is not necessarily canon, there is never any mention of him being a ghost. Mudou is also stated to have become an Ayakashi after one of his deaths (where normally he would revive as a human). As with Kaguro, he appears to have chosen this fate due to a desire for greater power.
 * Ghosts Who Linger, Are Destroyed, Or Pass On
 * Ghosts Who Become Ayakashi
 * Spirits Who Wander Endlessly
 * Ayakashi Directly (Without Becoming Ghosts First)