The unnamed Babysitter is an unseen villainess mentioned in "Overload," episode 2.03 of CSI (airdate October 11, 2001). She is a villainess from Nick Stokes' past, as she worked as Nick's babysitter one night when he was nine years of age.
The episode had Nick and Catherine Willows investigating the death of 14-year-old Dylan Buckley, and it was during their encounter with Dylan's therapist, Dr. Leigh Sapien, that ended up revealing that Leigh was engaged in an illicit relationship with a 17-year-old patient. Catherine noticed that Nick was fired up over the case and Leigh's reveal, and it was at that moment that Nick revealed that he was a victim of molestation when he was nine.
On the day in question, Nick's mother was working later, and a female babysitter was sent on the last minute to watch over him. The babysitter later turned heel and molested Nick, which left him instantly traumatized. As Nick also revealed, he remembered sitting in his room after being sexually abused by the villainess, as he was eagerly waiting for his mother to return home. It was never revealed if Nick informed his mother about what occurred, and the fate of the evil babysitter also remained unknown.