Mrs. Baylock was one of the apostates of Satan who knew that Damien Thorn was the Antichrist. When Damien's nanny Holly hung herself, Mrs. Baylock attempted to replace her as Damien's nanny as to protect Damien.
She was an Irish nun, known in the Roman Convenance of the Apostates of Satan as B'aalack.
When the byblical signs appeared to announce the coming of the Antichrist, she was working in Spiletto's hospital under the name of Sister Teresa.
While Spiletto chose the family for the adoption, and Brennan killed the biological mother of the Antichrist and the Thorns' new baby, her duty was to follow the impregnation and the birth of the Antichrist by the jackall and to prepare the Antichrist by shaving and cleaning him to make him presentable to the Thorns.
The day after Holly's death, Mrs. Baylock visits Robert and Katherine and successfully gets the job as Damien's new nanny. The day after Mrs. Baylock is hired, when Kathy decides to take Damien to a wedding with her and Robert, Mrs. Baylock (knowing that Damien will fear the church where the wedding is taking place due to his status as the Antichrist) tries to change her mind; but Kathy curtly orders her to get Damien ready anyway.
That night, when Mrs. Baylock brings a Hellhound into the house as a watchdog for Damien without Robert or Kathy's consent, Robert scolds her for it and orders her to have the RSPCA remove the hound in the morning. One day, while Robert is away at Dr. Greer's, while Mrs. Baylock is watching Damien play on his tricycle, she allows him to ride it out into the hallway. There, Damien rides his tricycle into Kathy, causing her to fall from the second floor. Kathy is subsequently hospitalized. The next day, when all of the Thorns' servants except for Mrs. Baylock leave, Robert has Mrs. Baylock look after Damien while he is away in Italy investigating Damien's origins.
One night, while Robert is away, Mrs. Baylock visits Kathy in hospital and murders her by throwing her out through the hospital window.
The next night, when Robert returns from Israel with the Daggers of Megiddo and finds Damien's 666 birthmark while the latter is sleeping, Mrs. Baylock wakes and attacks him to protect Damien. However, Robert is able to stun her by kicking her head against a drawer. When Mrs. Baylock recovers and finds Robert trying to escape with an unconscious Damien, she attacks him again and tries to stab him with a carving fork. However, Robert manages to stab the carving fork and a screwdriver into Baylock's neck, killing her.
Robert disliked Mrs. Baylock for making decisions involving Damien without his or Kathy's consent. Mrs. Baylock later viciously attacked Robert and attempted to kill him in cold blood to protect Damien.
Kathy was annoyed by how Mrs. Baylock was making decisions involving Damien without her permission. Mrs. Baylock later murdered Kathy in cold blood.
Damien and Mrs. Baylock apparently got on well, and Mrs. Baylock was quite caring to him. Mrs. Baylock also sacrificed her life trying to protect Damien.
Mrs. Baylock was involved of the Roman Satan Convenence guided by Father Spiletto and took part in the Irish religious war between Catholics and Protestants for him, organizing a terrorist attack in a market. After that she started working in Spiletto's Hospital under the name of Sister Teresa, and chosen with Brennan to be one of the three Servants of Hell to take care directly of the birth of the Antichrist.
Mrs. Baylock and Brennan worked together in the night of the birth of the Antichrist, having both been chosen by Spiletto for the task.
Her true identity was known to Bugenhagen, who revealed it to Thorn when we went to him in Israel.
- In a deleted scene, Mrs. Baylock survived being stabbed in the neck and chased Robert out to his car. Robert then killed Baylock by running her over with the car.