I wrote up some ideas for a Witch Hunter a while back.
My concept was that he was, essentially, a character who made the unit he joined a bit better. He had access to some level of ward save, but not the armour of other characters. He also had some tricks against enemy characters.
The big thing about him was the ability to motivate his unit each turn. At the start of each turn, the player declared what ability the Witch Hunter was trying to use and made a Leadership test (Ld 8 Hero). If successful, the unit got flaming/magical attacks, frenzy, stubborn, or whatever other ability he was granting. If failed, then...his rousing speech was less than rousing. It rendered him different from the other characters, but left the Captain as the real fighter, the Priest as the anti-wizard, and the Engineer as...well, still useless.