Robbie is Mike's brother who Fiona starts an affair with while dating Mike. He is irresponsible and lives life carelessly, even after being released from rehab, which he went to for alcoholism. He fuels his lifestyle by leeching off Mike and his parents, but justifies his manipulative actions by saying that he cannot live a "normal, boring" life with a desk job like his brother. Mike finds out about the affair at a family occasion and punches him in the face. His actions in Fiona's life cause a ripple in everyone else's lives around her, making him the main antagonist of season 4.