Description
Mari Gwyn is a pure white unicorn. Her body is somewhere between a horse and a deer, with her head being especially deer-like with a horse's mane. Her tail is long and thin with a tuft at the end, quite unlike either creature. The horn that crowns her head is straight, spiraled, and pearly. Her eyes are a pale blue.
A fine silver chain hangs about her neck, and on it an amulet of graven silver set with a clear crystal. This artifact allows her to take an alternate form, that of a pale-skinned woman of indeterminate age with stark white hair -- the artifact itself manifests as a normal necklace, and generates a simple white dress. If the artifact is removed or destroyed, Mari reverts immediately to her natural form. She also may revert to her natural form at will.
History
Mari Gwyn (A name that means "White Mare", given to her by one of the first humans she had a friendly meeting with) was once a normal unicorn -- on her plane, this means a quadruped of which she is an average example, possessing a natural aptitude for white magic, a life span in excess of a hundred years with no degeneracy in old age, the ability to communicate telepathically, and most commonly a demeanor that has earned the unicorns a reputation as holy creatures. Due to their telepathy, they do not possess personal names -- the concept of 'I' is sufficent enough to communicate the unique self
One day, a stranger arrived: a human Planeswalker. The Stranger set up operations near where the unicorns lived, and eventually began harvesting them for some unknown ends: perhaps to extend her life, though she suggested that there was some higher, more necessary purpose than that. Mari ended up discovering and killing her, but the dying planeswalker transferred a fragment of her soul into Mari. Mari believes the Stranger transferred her spark in that interchange, but it is far more likely that the graft of memories simply ignited Mari's own nascent spark.
For some time, Mari wandered the planes in fear, for she found very quickly that the strange planeswalker was far from the only being that would hurt or kill a unicorn for personal gain. Eventually, she met Cornelius -- a mage with quite a bit of sadly out of date knowledge regarding planeswalkers -- who was able to give her the artifact that lets her take a human form. More free then to interact with the planes, she has been searching for a way to go back to what she was before ever since.
Personality
Mari Gwyn is curious and hot-tempered, at least for a Unicorn. She is still is currently looking for a cure to her Planeswalker "Curse", but as she examines and learns from the memories of the Stranger, it's possible she'll discover a greater reason for her current state. Despite her abiding desire to return to her home and once again be just a unicorn, there are parts of her that feel a great thrill at traveling Dominia, and might be comfortable with it were it her own choice to do so.
Mari is also rather sweet -- quick to make friends, and generally quick to make amends. Few, terrible things will cause her to hold a grudge for very long. She is also, to her disadvantage, an honest creature. Though the artifact she possesses lets her pass among normal folks quite easily, she hates having to use it to deceive and has occasionally been too free with the truth of what she is, though she is gradually learning to hide.
Powers
Mari Gwyn is a unicorn, which entails a natural talent with white and green magic, especially magic of healing. She's still not much of a spellcaster, but being a Planeswalker has amplified the minor power she had, allowing her a small array of protective and curative spells. Mari can speak telepathically, and is thus able to communicate with any creature that is known to her, within about fifty feet, and that posses a language. She cannot read minds, only speak mentally and understand replies made in any language.
If she takes on her human body, her abilities change a little -- her telepathy is deadened, requiring physical contact. While she retains her natural healing and protective magic, when in human form she can actually utilize a tiny bit of the Stranger's magical knowledge. If she wanted practice such spells, she could probably cast them as a Unicorn, but as it is the fact that the Stranger was human means that her memory has an easier time translating over when Mari shares a human shape.
Notes
* The technical power of Mari's amulet is to allow its wearer to "take a form that reflects their inner self", with the caveat that the form is always humanoid, and decently clothed (after failures of shape and modesty for its initial wearer resulted in alterations). Mari's particular shape reflects both something of her self-image (the color of her eyes, hair, and dress) an some of her character traits: for instance she is soft and kind, so her human form is delicate and not physically imposing. So, if taken by someone else, it would not give them Mari's human form, but an alternate, humanoid shape of their own with features somehow fitting to their core personality traits. As the artifact's maker was likely of the white/green persuasion, it is likely that those with a similar ethos would receive the best results.
* Briefly mentioned in this dossier and more detailed in Advent of a Planeswalker is the human Cornelius -- once Spellsquire (or, "a lot of things") to a pre-mending planeswalker, he seems to have been rendered immortal, or at least immune to age, some time in the distant past.
* Mari appears in "Lost Things", set in the M:EM's present, wherein she meets Aria. She also appears in "Advent of a Planeswalker" which roughly details her background from the arrival of the Stranger among the unicorns to her gaining the amulet.
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planeswalkers and much fewer 
planeswalkers, that I'd like her to be both white and green. Prized Unicorn and Zebra Unicorn are green so it wouldn't be strange.
