Kaya scouted ahead and it looks like the coast is clear for now.”— Search party log #1
She was one of several members of a search party whose purpose was to find the party leader's sister, Cheryl. She was often tasked to scout an area, helping the search party avoid any serious confrontation. After a lone BoS scout gets killed in action (and presumably buried just west of the Sewer Waystation), Kaya refuses to go any further and supposedly heads back to Canterbury Commons, likely leaving Manny on his own.
Her fate is unknown, as she's never seen or heard from again, however she is believed to be the only remaining survivor of the search party.
Kaya is only mentioned in Fallout 3.