Living receptacle of Asari Law
The Justiciar is the fate of all Firsts. They are literally the embodiment of Asari Justice.
Each Justiciar serves the same term of service as the First, and is then replaced by the First when her term ends. On the day of the new First’s inaguration, the old First and old Justiciar Join Minds. When this happens, the old Justiciar dies and the new Justiciar has the consciousness of every single previous First and Justiciar that has passed.
The asari body is generally incapable of maintaining this consciousness for longer than a decade and, by the end of their time in service, are so thoroughly exhausted from maintaining their personality under the onslaught of the dead that they willingly surrender their lives to pass the burden on to another.