She ran away from the other survivors after Randall Clark's one-man, year-long, guerilla war on the survivors. On January 13, 2097, she stepped on one of Randall's bear traps at Cueva Guarache. Her screaming woke him up, and he soon found her. The way she was crying moved Randall, especially after she spotted Randall, who had been the survivors' boogeyman. He freed her and assumingly tended to her wounds.
After confirming her story (matching the "interrogations" of other captured survivors the year prior), they started a relationship with each other. She did not know how to survive outside of a vault, but wanted to learn. By September 9, 2100, the relationship blossomed and she was pregnant with Randall's child. However, Michael was born breech, and eventually died due to complications during delivery. Randall gave Sylvie anesthetic to put her out while he attempted a Caesarean section. He was too late and Sylvie never regained consciousness, dying along with their child, Michael. He buried them both south of the Narrows. Being by their sides comforted him, as he never got to say goodbye beforehand.
Before her death, Sylvie was a trusting and religious person, specifically referring to the Vault 22 survivors as "children of the devil" and saying that it was God's will that they would have their child. Randall never told her about Charlotte or Alex, not wanting to tell his young wife about the previous one who he failed.