For all we know the figure in the artwork was just some abstract soul-like form that's now put inside a bonecrafted body.
That being said Katakros must be one hell of an impressive guy considering what Kelly said in the interview. When Nagash was about to destroy his city Katakros went to Nagash and offered to work for him ... and instead of dismissing/smiting/eating/whatever the foolish mortal, Nagash actually looked into his achievements and went "Yup, you are the real deal. Be the Mortarch of my pet project legion".