Sean, played by John Benjamin Hickey, is Cathy's outlandish and whimsical brother, who is selectively homeless, and is unreserved when it comes to pretty much anything he thinks and does. Sean Lives his life on his own terms and is more than happy while doing it. Sean is and always has been for ungovernable freedom and is not ashamed to make it known. After Cathy finds out about her diagnosis and is ready for her "life change", she finds herself seeking advice from her brother, which is ironic as they have always been opposites.
When Cathy finds out about her melanoma, she is reluctant to tell her brother because she's afraid of how he will take it since their mother died of cancer while they where young. He has a fling with Cathy's old friend Rebecca, who later tells him that she's pregnant. Cathy decides to give him Marlene's house so that way he can prove to be a responsible father.
Sean is now living in Marlene's house with Rebecca, but has yet to learn of Cathy's cancer. When he does find out, he yells at Cathy for her selfisheness since he was the last person to know. He then decides to get on medication for his bipolar disorder in order to be there for his sister, and the rest of his family. Rebecca learns that the baby will most likely be a girl, so she and Sean decide to name her after Cathy. He and Rebecca continue to grow closer, and decide that they want to get married before the baby's born. But Rebecca ends up losing the baby at the end of "The Last Thanksgiving". Following the miscarriage, Sean goes off his medications in order to experience the loss of his unborn daughter, "baby Cathy". He begins to act erractically; causing Rebecca to leave after the baby's funeral. Sean ends up running away after a couple with a newborn move into the neighborhood. He returns in the season finale. Claiming to have met and joined a traveling circus, but that he left after someone tried to cut his toes off in his sleep.
Sean get's a job as a janitor at the same school where Cathy teaches in order to pay for his house's utility bills. He's still upset over Rebecca's depature, and burns a box of tampons that she left along with Andrea. After getting a landline, he discovers that the person who previously had the number was a successful gay sex phone operator. Sean uses the line to make more money. Despite some difficulty when the previous phone owner showes up and starts harassing him. He becomes friends with a regular customer named Tim. He then starts a relationship with him and his wife Giselle, but they break it off when Sean's presence begins to negatively affect their marriage. He accompany's Cathy, Paul, Adam, and Andrea to Puerto Rico where he leaves Cathy alone while scuba diving to hook up with another woman.
Wanting to do something meaningful and life affirming in light of what is happening to Cathy, he decides to donate a kidney to an unknown recipient. He and that recipient, Ray, decide to meet days before the surgery. In the simple act of Ray offering Sean a gift of an expensive Rolex watch in gratitude of what he's doing, Sean can see that Ray is everything he abhors in life. Although Sean decides to go through with the donation on Cathy's urging, he also decides because of Ray that he will return to being homeless again.