Robbie is Mike's brother with whom Fiona starts an affair while dating Mike. Robbie is responsible for Fiona's life taking a drastic turn, making him the main antagonist of Season 4.
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 his brother 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.
During one visit in"Like Father, Like Daughter", Robbie goes out for drinks with them and mike ends up drunk. Robbie talks with Fiona before suddenly kissing her, the two have sex but the encounter is broken up when Mike nearly catches them. Robbie blackmails Fiona into an affair and while she is upset, she enjoys the wildness.
In "There's the Rub", Robbie argues with his brother at Fiona's birthday party while at their parents' home. Robbie tells Mike about the affair, prompting Mike to give him a well-deserved punch to the face. Mike then dumps Fiona. The episode ends with Robbie giving Fiona a baggie of coke for her birthday.
Robbie learns from Mike that his actions in Fiona's life have caused a ripple in everyone else's lives around her, after her brother Liam is sent to the hospital. He arrives at the Gallagher Household to sincerely apologize to Fiona, who is outraged to see him and he gives her treats as she tries to slam the door on him. Robbie stops her and they talk for a while, he asks if she ratted him out for the incident but she replies that she didn't. Fiona curses him and closes the door in his face, Robbie wasn't surprised by this reaction and walks away, feeling guilty.
Eventually, Fiona goes to him after violating her parole and is left unconscious but found by her siblings. Debbie tells a hungover Robbie to stay away from Fiona and threatens him,