Catherine Walker is a Conservative front-bencher designated by her party to demolish Ros in debate, but who instead defects to Ros's Purple Alliance. She becomes Deputy Prime Minister and Chancellor of the Exchequer. As a young woman at Oxford University, Catherine had the chance of a fellowship to study at Princeton University but her then-tutor Hilary Rees-Benson (later a political opponent) recommended another student instead, in part due to jealousy of Catherine—the two remain rivals even when serving together in Ros's cabinet. Catherine was formerly in love with Conservative Party Leader Paul Critchley, but he cheated on her and dropped her. While Chancellor, she conducts a sexual relationship with her speechwriter Ben Sixsmith—21 years her junior—becoming pregnant by him but terminating the pregnancy.