Sally is a young girl that is ecountered in the Mothership Zeta DLC. She if first found trapped within a cell near where the player begins the quest. After releasing her, she will guide the player around the ship. Sally knows the ship in great detail because she has apparently escaped numerous times to explore the space craft.
Sally provides the player with information for quests and also aids you by opening doors and telling you where you need to go. When the player completes the Mothership Zeta storyline, Sally will stay onboard the ship. The player can return and talk to Sally, who will give the player any items she has found while exploring the rest of the Alien craft.
The player can learn that Sally was born before the bombs fell on October 2077, making her age somewhere over 200. It is not known how she hasn't seemed to age, although she has most likely spent a long time cryogenically frozen. She will tell you that her parents perished along with most of the world when the bombs dropped in 2077. Sally aparently was abducted along with her sister a short time afterwards. The Aliens killed Sally's sister when operating on her. Sally tells the player that she watched the Aliens as they killed her sister.