Sister Regina is the main villainess from "Sister Michael Wants You," the first season finale of Diagnosis Murder.
She was played by Concetta Tomei.
Sister Regina is a nun at Saint Clair's nunnery school, but she was also in league with the Strega crime family and the corrupt Councilman Fletcher. She shot and killed Greg Wilson, a cameraman who was recording a promotional video for the school, with the video most likely also containing incriminating evidence. The murder led to Sister Michael bringing in Mark Sloan to pose as a priest to investigate, but during their meeting with Steve Sloan in Sister Michael's office, Sister Regina figures out the ruse and has Mark and Steve thrown out, all the while berating Sister Michael for allowing men inside the school.
After Mark and Sister Michael find the incriminating tape, the evil Sister Regina appeared and pointed a gun at the pair, revealing herself as Greg's killer. Mark later shuts off the lights before he and Sister Michael hide out, while Sister Regina fires shots wildly. She is later distracted by Mark, who bangs the organ, which allows Sister Michael to knock out the villainess with a right hook. Sister Regina is arrested for Greg's murder, and she turns against her co-conspirators as she's taken away.