Rennifer Longwaters is the chief undergaoler of the dungeons beneath the Red Keep. He is a bowed old man. He is descended from Ser Jon Waters, the bastard son of Princess Elaena Targaryen and Alyn Velaryon.
A Feast for Crows
Ser Jaime Lannister enquires into Rennifer's role in the escape of his brother, Tyrion Lannister. Rennifer explains how the cells work, and boasts of having royal blood. When Ser Jaime seeks out Ser Ilyn Payne before departing for the riverlands, Rennifer lets Ser Jaime into Ser Ilyn's quarters.
—Rennifer to Jaime Lannister