On Criminal Minds
Sarah was once Gideon's schoolmate but lost contact with him at an unknown point. Sometime prior to The Fisher King, Part 1, she reunites with Gideon and the two of them set up a date at Gideon's cabin. There, they have dinner and dance together before Gideon receives a cardboard box that contains a man's head from an unidentified sender. The shock of it caused Sarah to throw up and she is presumably brought back home by Gideon. She isn't seen in the rest of the episode.
In No Way Out II: The Evilution of Frank, she meets her demise when she sets up another date with Gideon. As Gideon looks for flowers to give her, Sarah (who was at Gideon's apartment) answers the door for Frank, believing it is Gideon, only for her to be injected by ketamine and killed by Frank. Distraught by Sarah's death, Gideon eventually tracks down Frank just in time to see him and his own girlfriend Jane commit suicide.
In Doubt, a guilt-ridden and traumatized Gideon begins seeing hallucinations of Sarah while investigating a case in Flagstaff, Arizona. These hallucinations combined with the stress and guilt he is experiencing eventually caused Gideon to resign from the BAU for good. She is later mentioned in Nelson's Sparrow, with a photo of her being seen in a notebook Gideon kept after departing the BAU.