Mary Reese is a Catholic school student who lied to her father Jacob that teacher Charles Nash sexually abused her, resulting in Jacob killing Nash. She was actually being exploited by Luke Thornton, the school’s counselor.
Mary tried to perjure her testimony to protect her father, lying it was Nash’s refusal of more conservative academia in his classroom that set Jacob off. When the truth came out, Mary was threatened with charges of perjury if Jacob didn’t plead guilty. (L&O: "Good Faith")