Type: Null is a quadruped, chimeric Pokémon with traits from a variety of creatures. Its head is enclosed in a brown helmet with green markings, designed to suppress its power to stable levels. The helmet has a large jaw piece that wraps around the front with large, round hinges on the side. Each hinge is covered by a green X-shaped marking. Small gray eyes can be seen from within rectangular black holes on the front, and there are two pointed ears on the top. Around the base is a collar with four protruding vanes that are tipped with a three-way spike. This collar has a thin green line between the vanes, and each vane has a green hexagon near the end. An axe-like blade with a band around the middle extends backward from the top of the helmet. The front tip of the blade has a small, silver spike that hooks downward.
A mane of gray fur hangs from below Type: Null's helmet and covers its shoulders and chest. Most of its torso and legs is covered in short black fur. The lower half of each front leg is insectoid and green with spikes along the back, while the rear legs have thick purple scales on the flanks, shins, and on the top of the foot. All four of its feet have three toes, but in the front two toes face forward and one backward. Additionally, the hind paws have a short, purple claw on the back of the ankle. Its fish-like tail consists of two dark blue spines with a light blue fin stretched between them. The top spine has a line of short, gray spikes.
Type: Null was created by Faba of the Aether Foundation to be an "Anti-UB Fighting life-form". Codenamed "Beast Killer", Type: Null was given cells taken from all known Pokémon types. This was done with the intention of giving it the ability to shift between types by holding a corresponding memory, an ability originally inspired by the myths researched at Canalave Library. This ability led the Aether Foundation to give it the name Type: Full (Japanese: タイプ：フル Type: Full). Three known models of Type: Full were created, but all three rejected the RKS System and went berserk. After being subdued, the three Type: Full were given limiter helmets to lessen the effects of the RKS System's rejection and were put into permanent cryogenic stasis, and the "Beast Killer" project was deemed a failure, and their name changed to Type: Null. At some point before the events of Generation VII, Gladion was able to free one of the Type: Null, which he adopted as his own in the hopes of finding a way to undo the effects of the limiter helmet and allow it to use the RKS System.