The Sin Hunters (also known as Mithran Trackers) are a group charged with enforcing Mithran law.
The current known members are:
When fought, they process enmity separately, but skillchain together on whomever has the most hate.
A fourth known member is Shikaree M who is involved with the Ranger af quests.
A fifth known member is Shikaree G, who arrived in Mindartia during the Crystal War in pursuit of Lehko Habhoka. She is the mother of X Y and Z according to Koh Lenbalalako.
They appear to be FFXI's version of the Magus Sisters, recurring characters in the Final Fantasy series.