Katy is Tommy and Janet Gavin's youngest daughter. Like her siblings, her love is usually for sale, depending on which parent pays the most. After the death of her brother Connor, Katy is for a brief time withdrawn and existential. Her dark outlook on life and lack of belief in God scares Sean Garrity. Katy throughout the series becomes more hard-nosed towards her parents, especially Tommy when he relents on retiring from active duty in the FDNY.