Derek Reese helped with Sydney's birth; after her mother died, her half-sister Lauren Fields was her only surviving relative. Derek offered to let Lauren and Sydney stay with them in his safe house, but when his back was turned, Lauren slipped away with her baby sister.
Sydney survived Judgment Day along with her sister. She was stuck in a Resistance bunker when Skynet unleashed a biological weapon on 200 Resistance members. Sydney was the only survivor, and sent out an SOS. Derek Reese and Jesse rescued her and brought her back to Seranno Point Nuclear Power Plant.
Derek and Jesse were administered her blood to fight off the poisonous biotoxin in their bodies. Sydney's blood was used save the lives of many Resistance fighters.