Sister Monica is a character in the Season 3 episode titled "And a Child Shall Lead".
Sister Monica is a teacher at the St. Julian's School, which caters to children with autism and other developmental challenges. Among the students is Bobby Phelps, the son of aircraft designer Robert Phelps. At the beginning of the episode, Sister Monica comes to Santini Air to remind Dominic, Caitlin and Hawke about their promise to help out at the school picnic. She also tells Hawke that Booby told her he had "a deal" with Hawke and to be sure he is not disappointed.
Later, Bobby's father is kidnapped by a gang of terrorists. When Hawke realizes the gang might be after Bobby next, he and Caitlin rush to St. Julian's where they meet Sister Monica. She tells them some F.B.I. men had come to take Bobby away, saying that they had secured his father's release. In actual fact, these were members of the terrorist gang.