A tree spirit is a humanoid being, sometimes classified as a member of the fae, and other times considered a close relation of the nymph; perhaps a crossbreed of the two, though most commonly tree spirits are regarded as a species of their own. Tree spirits gain their name from the fact that they inhabit, and may perhaps be the soul of, a tree. They are almost always female, and typically bear resemblance to a small child, though with bark-like skin and covered in foliage. Tree Spirits are indigenous to Western Engelain.

The trees which a tree spirit inhabits tend to be solitary, though the reason for this is unknown. It is suspected that tree spirits reproduce similar to tree spawning through the spreading of seeds or perhaps pollen, though it is not known why tree spirits grow in areas where regular trees do not also sprout. In this way the home of tree spirits are easy to identify, as their tree will be the only tree of its kind in any given area. Sometimes a number of tree spirits may group together, though this phenomena is rare.

The link between the tree spirit and its home tree is only partially understood. It is known that the tree spirit does not choose to which tree it is bound, and presumably is born with the tree, or else later develops within the tree after it has reached full bloom, (a tree spirit has never been seen with a home tree that had not yet reached adulthood.) It is known that the destruction of the home tree kills the tree spirit, and the killing of the tree spirit in turn causes the home tree to die out. However, whether the tree spirit is the literal soul of the tree or simply a symbiotic being is unknown.

Tree spirits are sentient and capable of communicating in the common tongue. They have a friendly and jovial demeanor, similar to the child-like forms they take. It is suggested that tree spirits mature with the age of their tree, and there have been some infrequently reported, (and therefore somewhat doubted,) sightings of tree spirits that have reached adulthood. However, given the prolonged lifespan of a tree, it takes numerous centuries for a tree spirit to be witnessed as maturing, and their lifespan is speculated to be in the thousands.

Although often compared with nymphs and having numerous similarities with them, there are some distinct differences that lead many to believe tree spirits are their own separate entity. First and foremost would be the fact that a tree spirit appears as a child, and research of particular tree spirits over the timespan of several centuries have noticed minute signs of aging. Nymphs on the other hand are born in adulthood, and are ageless. Nymphs also always appear without clothing, which tree spirits are covered in foliage. Tree spirits are also capable of, with great difficulty, uprooting their home tree and allowing it to move. This technique is used sparingly however, as it is said to cause great pain and will lead to disastrous consequences in the future.

Similarly the tree spirit is oftentimes lumped in with the fae, though there are some who believe this to be erroneous. Tree spirits tend to be somewhat larger than most fae, though the main differentiation is their non-corporeality. Though tree spirits have a solid, corporeal state, they can also when threatened dissipate into leaves and pollen, where the non-corporeal spirit may retreat into the safety of their home tree. Similarly, a tree spirit can slip into their home tree as easily as a ghost slips through a wall. They can often conjure up copies of themselves using fallen leaves and communicate in this method, if wary of the company that has arrived at their tree.

Though tree spirits can communicate with mortal races with ease; this does not aid much in the study of their kind. Given that most known tree spirits are childlike, they are simply lacking the knowledge, or else do not have the attention span to care for such questioning, preferring to play games instead. As such, tree spirits have lasted throughout most of history without ever making any kind of impact, though experts suspect that the number of tree spirits is in fast decline.

Tree spirits can travel from their home tree, though only for a short distance. It is unknown what happens if a tree spirit tries to go further than this limit, whether it is a literal limit of their abilities or a self-imposed radius around their home tree. There is wide debate as to whether or not humankind can reproduce with tree spirits, given that there is so few tree spirits of adult form that would be considered at reproductive age. The mythic Engelite figure of Arboretum Salix, Lord of Entinium, is sometimes said to have a tree spirit for a mother.

Given that most sightings of male tree spirits have turned out to actually be somewhat boyish females, tree spirits are assumed to be an all-female species, reproducing asexually.