Katelyn along with other Gur can be encountered in the Gur Camp attending a funeral for the fallen from a recent offence on Cazador's palace. Katelyn with her fellow Gur, all members of a group of monster hunters lead by Ulma, are looking for their children, who have been kidnapped by Cazador's spawns. The nomads have camped in Rivington, trying to mount a rescue or seek revenge.
Ulma asks the party to help in defeating Cazador and discovering the fate of their abducted children.
Once Cazador is defeated, Katelyn Lerskin, along with Ulma and the other Gur can be found near the waypoint in Cazador's Dungeon. She attacks the party if the ritual held in the dungeons was completed.
If Astarion had been turned in to Gandrel in Act One and kidnapped, then in Act Three Katelyn and all of her fellow Gur (except Gandrel) are dead.