Well, there might be the remains of what used to be a body inside that outfit. But his mind is already contained in the armor he is wearing, and possably the mosk mostly. So the entitty known as Nihilus is really mostly armor, outfit, and a mask, with some dead human remains inside it. Imagine how this would smell.
Well, its like ... well, I am not going to quote warhammer40k here, but you know what I am saying.