The Mechanist Helmet Guard is a Mechanist wearing a full head helmet that protects him from being knocked out. The Design is very much like the Standard Karras inspired design with the Eye holes and Mouth open.
Only two of these appear in Thief 2: one in the level Trail of Blood, patrolling the gathering place in Beck o' the Wills, and one in Kidnap as a bodyguard for Brother Cavador.