- "A legendary sword, which once belonged to Jack's father, Cairn. It has very strong powers but Jack can't use it yet."
- —Important Item Details Window
- "The decision maker's sword. It has no special powers but it is stronger even than the weapons of the four spirits."
- —Weapon Details Window
The Arbitrator was the sword used by Cairn Russell to defeat the Water Dragon sixteen years before the events of Radiata Stories. Its name refers to a person chosen to decide a dispute or settle differences, especially one formally empowered to examine the facts and decide the issue. In Radiata Stories, it is Jack who settled the differences between humans and non-humans, making him the active arbitrator or deciding factor in the game. However, Jack cannot actually use the sword for most of the game because of the taint of Algandars that the blade carried with it, due to Cairn's defiance of the cosmic order of Tottaus.
On the Human Path, Jack never encounters Gawain outside of combat, and as such, is never sure enough of his abilities to use Arbitrator. He is seen with it when he enters Radiata for the first time, as well as when he departs from Radiata during the ending, carrying it along with a knapsack.
- Cairn's version of the Arbitrator is larger than Jack's and seems to function as a double-handed sword instead of a one-handed sword as seen in the battle with him in Distortion Corridor.