Fantasy Shadow is almost entirely dark grey in colour, save for a few light grey spots and its green and grey eyes. It has two primary forms, one being anthropomorphic and the other being a small, flying head. Its face in both forms consists of a long, cone-shaped nose, a concave crest on top of its head, and two very asymmetrical eyes; while Fantasy Shadow's left eye is small, circular, and green, its right eye is semi-rectangular curved at the end, light grey, long, and divided into eight segments. Both eyes lack pupils. Although it rarely opens it, Fantasy Shadow's mouth is filled with long, spike-shaped teeth that are the same dark colour as the rest of its body.
In its anthropomorphic form, Fantasy Shadow has a very thin torso, very thin neck, and six equally thin limbs, four of which, appearing to be arms and legs, which are cone-shaped, with the ends of the limbs being flat and broad, contrasting with Fantasy Shadow's predominantly thin physique. These limb ends are stated to be hooves. The other two limbs, appearing to be wings, end in curved spike shapes. It also has a tail, which, while mostly as thin as most of its body, ends in a three-spike tuft. Its tail and lower two limbs are connected to a relatively compact pair of hips, while its neck and upper four limbs are connected to an even smaller, rectangular prism-shaped chest.
In its flying head form, Fantasy Shadow is teardrop-shaped. Aside from its face, Fantasy Shadow's only remarkable feature in this form is a single, thin appendage ending in a curved spike.
Fantasy Shadow's backstory is unknown; it mysteriously appeared one day to begin following Tenguman. Eventually, after a long time of following Tenguman through a desert, it lead him into a western-styled town, where he was shown disturbing visuals before being tied up and shut down. Exactly how much Fantasy Shadow was involved in this event is unknown, but Fantasy Shadow's reaction implies that it intended to shut down Tenguman.
Later, when LaV came to Tenguman's inactive body and possessed it, Fantasy Shadow also entered Tenguman's body, agreeing to share it with LaV. Eventually, Fantasy Shadow, using Tenguman's old body, confronted Tango and challenged him to a "dance," intending "to beat [him] into the ground with [his] own ridiculousness." Tango then performed Josuke's pose and punched Fantasy Shadow, causing its body to revert to normal. He then informed Fantasy Shadow that he made contact with his older body and used said contact to revert Fantasy Shadow to revert to its original form. Tango then promptly performed Kira's pose, quickly destroying Fantasy Shadow outright.
Fantasy Shadow is able to switch between its anthropomorphic and bodiless forms at will. It can apparently also take on other forms, as it was able to alter its appearance to resemble Tenguman, even before possessing his body. In its flying head state, it is able to not only levitate, but also possess empty bodies such as Tenguman's.
At one point, Fantasy Shadow says "With said holiday right around the corner... Maybe I should drop by and pay everyone a visit?" Given that "said holiday" is referring to Hearts and Hooves Day, this quote suggests that it may have pony relatives or even be some variant of pony itself.