Mary Davis was a Dee Valley University Hospital patient who was admitted for a blood test at the same time as Maxine Minniver. Maxine, suffering from Munchausen Syndrome, switched the blood results. Mitchell Deveraux received the results and found that "Maxine" had high levels of C-reactive protein. Maxine later saw Mary get discharged from the hospital.
Mary only appeared as an extra in Episode 5162, having no dialogue and was therefore uncredited.