Kurisu is a young tribal girl kidnapped by slavers. She is also the sister of Sulik, a tribal warrior. Although Sulik's quest was to find her, he never did. She somehow appeared many years later somewhere in the Southwest and became leader of the Blackfoot tribe.
The Chosen One and Sulik were meant to search for the latter's sister, who had been kidnapped by slavers. Unfortunately, this quest was cut and the slave camp along with Kurisu herself are nowhere to be found.
The Prisoner can make her a companion if they convince her to leave her tribe, this is only possible if there's a backup leader, (such as Chagas), available for the tribe.