Very tall, obviously robotic, somewhat skeletal in appearance. He was designed to be intimidating. Silhouette is broad (especially thanks to his shoulder plates) with a bulky chestplate and inhumanly long, thin limbs. Lower torso is a narrow, flexible spine with multiple pistons supporting the weight of his upper body.
Body language generally either very aggressive and direct or very apathetic and standoffish, depending on his mood. His posture is somewhat poor, with a tendency to slouch. When fighting or hunting down a target, his movements are extremely predatory and deliberate. When sneaking or stalking, especially when moving in a crouch or climbing around on walls or ceilings, he has been described as skittering, somewhat resembling a very large spider.
Eyes glow yellow, he does not have eyelids and so never blinks. The intensity of the light of his optics may change— for example it seems to flash brighter with strong emotions, such as when he's feeling especially murderous, or dim and flicker off briefly when in extreme pain.
Important to note is that he does not actually have a mouth, so his jaw and lips don't move when he speaks.
The only clothing he wears is a red cowl and bandana, and a red loincloth (presumably due to some lingering sense of shame from when he was human, because he sure has nothing to cover up down there).
For a visual example you can check out his intro in the season trailer
here.