Manhunters aren't pledged to any kingdom, and therefore don't fight in any wars except against bandits. The color of their units is quite similar to, but a little lighter than, the color of bandits. It is possible to increase relation with all manhunters by helping them kill bandits, providing they are about to lose, though relation with manhunters seems to have no impact on anything.
It is very handy to keep a few of these units in your party. While the majority of your forces will pick off most of the enemies, your horse-riding manhunters (who use blunt weapons) will be able to knock some unconscious, netting you a few prisoners. They handle themselves fairly well at higher levels.