Saralynn O'Brien is a minor hidden villainess of "Don't Stay Up Late", the 2nd book in R.L. Stine's revived Fear Street series (published on April 7, 2015). She was a Shadyside High student and the best friend of protagonist Lisa Brooks. The two girls became friends soon after Lisa moved to Shadyside, and it was through Saralynn that Lisa met her boyfriend Nate Goodman. But unbeknownst to Lisa, Saralynn and Nate were cousins of Brenda Hart, and were involved in her sinister plot to create a half-human, half-demon creature. The other members of their circle were Brenda's sister Alice and Dr. Shein, a psychiatrist friend of Brenda's.
Lisa was unwittingly roped into the plot when Dr. Shein (who Lisa was seeing after she was involved in a car accident that claimed her father's life) suggested she take a babysitting job for Brenda's "son" Harry, the successfully made creature. As Lisa began to witness more of Harry in monster form, Dr. Shein began trying to convince Lisa she was simply still in recovery for the trauma she experienced. Saralynn also assisted at one point when she went to the Hart household with Lisa. When Lisa saw Harry in monster form at the top of the stairs and tried to point him out to Saralynn, she lied and tearfully said there was nothing there (though whether she truly felt remorse or if her tears were simply to further deceive Lisa is unclear). Lisa first began to discover her friend's evil secret when she found a photo of her, Nate, and Brenda together in a photo album Brenda had.
In the end of the book, Nate rescued Lisa from Brenda when she attempted to kill her for uncovering Harry's secret. Afterwards, both Nate and Saralynn disappeared from town and avoided arrest. Harry later resurfaced at a daycare center Lisa began working at, but Saralynn's official fate was not revealed.