Arthur is a spirit; he actually has no gender but identifies mostly as male. His face is a near-featureless white mask that gives off a glow like moonlight, with wide, brightly glowing white eyes that almost never blink, narrow, or express any particular emotion. His form is thickest about his face and can be between three and eleven feet long, and is made of a vividly blue substance that is often in strands and tendrils, almost like hair. Arthur is cool to the touch, and can control many things about his being, such as how much he glows, how solid he is, and how transparent he is. In sunlight however, he becomes invisible and cannot make himself visible or solid; he basically has no way of interacting with the world in sunlight.

An alternate form Arthur usually takes when on the move is that of a small, blue, glowing sphere that leaves a tail of blue like a comet - the faster he goes, the longer the tail.
When he wants - which is rarely - he can manifest arms and hands.