Gretchen is Monk’s half-sister. She is the daughter of Fiona, a woman whom Monk’s father had an affair with while serving overseas. Monk’s father lost contact with Fiona and Gretchen after Fiona broke off the affair and ordered Monk’s father not to contact her. He wrote Gretchen a heartfelt letter but never sent it. Monk eventually tracks Gretchen down and introduces himself, but Gretchen receives him coldly—she has lived a rough life of struggle compared to Monk’s privileged upbringing and feels bitterness toward Monk’s father for never trying to find her. Monk writes Gretchen a check for $100,000 and claims his father meant for Gretchen to have it, though he knows it won’t undo any of the damage his father wrought.