Siobhan "Mrs. S" Sadler

Siobhan Sadler is the foster mother who raised Sarah Manning and her brother Felix, now a steely, watchful presence in Sarah's chaotic life. She goes by Mrs. S, speaks in a soft Irish lilt that never quite hides steel underneath, and clearly knows more than she says about the danger closing in.

Maria Doyle Kennedy plays her as warmth wrapped tightly around something harder, a woman whose maternal concern is real but never simple. The writing keeps doling out reasons to distrust her competence and her secrets, and Kennedy makes both the tenderness and the menace feel entirely sincere at once.