Carl Francis Gallagher is the second youngest Gallagher child. He shares a room with older brothers Lip and Ian, who try to keep their more adult interests from him. Carl has a few secrets of his own, including breaking every rule he can find. It’s safe to say that with parents like Frank and Monica, Carl could be screwed up even more. But it is thanks to his older siblings sacrificing their time, energy, and love to make sure that they are providing him with the best life they can.
In Season 1, Carl plays a minor role. He is shown to be a bit of a delinquent however, when he gets into trouble at school and possibly faces expulsion.
In Season 2 Frank's mother, Peg, comes to visit. Peg has ulterior motives other than just visiting her son and his family, but Carl ends up striking a relationship with her. She teaches him to play poker, buys him things from department store, even wants to help him learn a skill... making meth. She takes Carl with her to the local drug store to ask people going in if they could buy certain medications for his sick grandma, a young boy being a perfect cover. They get the items they need and head back to the Gallagher house and set up in the basement with their meth making operation. Peg needs to leave and asks Carl to keep an eye on the cooking. Leaving Carl alone was not a good idea however, because he ends up causing an explosion, rocking the entire house.
He has his head shaved during the third season due to lice, however Frank convinces him he has cancer (in an attempt to extort the Make a Wish Foundation). Instead, the Foundation sends Carl to cancer camp. Before leaving, Frank gives Carl an unmarked bottle of antacids and tells him they're pills to cure his cancer. Carl hates the camp, as everyone is treated with "kid gloves" and made to participate in wholesome activities. He gets to know another kid and decides to defy the rules set by the camp. A pretty female counselor notices that Carl is unhappy and tries to help him have more fun, but her activities don't live up to Carl's expectations. At night, Carl tries to break into a cabinet with a new friend and another female camper, when the female counselor catches them in the act. Upset that she has failed in her duties as a counselor, she begins crying and asks Carl and his friend how she can make their camp experience more fun. In response, Carl tells her he wants to see real life boobs before he dies (still thinking he has cancer). The counselor complies and shows Carl and his friends her breasts, but is caught by the other counselors, and ultimately fired.
Carl also has an unhealthy fixation with death, and goes so far as the attempt to kill his uncle using rat poison in the middle of the third season (after the uncle tries to evict them from the family home). Fortunately, the uncle survives, and Carl's biggest offense is attempted murder, rather than first-degree murder.
Since Carl is still a kid, he has no understanding of sex and sexuality, but has a strange fascination with homosexuality and what exactly it entails. Often in the third season he asks members of his family, and his friend Little Hank about gay sex, simply out of curiosity. Ironically, he and Liam are later placed in a foster home with two gay men interested in adopting them. While staying at the foster home, Carl discovers the couple's gay porn collection and is caught watching it in the couple's bedroom, to which he asks "why do they all have mustaches?'
Carl eventually returns back to the Gallagher home later in season three after Fiona is granted full guardianship of the kids. Later, in an attempt to bond with his father Frank, Carl uses the gay couple's security code to break into the foster house with Frank and rob the place. After Mickey's wedding, Lip and Ian come home to find a police officer arresting Carl for the crime. In a shocking stray from character, Frank appears at the scene and confesses to the crime, forcing the police to let Carl go.
Carl's interests include drinking, weapons, explosives, and death. Deep down, Carl is actually a good kid messed up by a completely dysfunctional childhood. He cares deeply about his family, especially Frank, despite Frank's complete and utter failure as a parent and role model.
In season 4, when Frank's liver fails, Carl takes it on himself to find him an organ donor. Carl later meets a girl named Bonnie, who lives with her family in a van. They scheme together to gather money for Bonnie's family, and Carl slowly falls for her. He denies this however, when she asks him about it, to which she replies she doesn't want him to love her. The next day, the van is gone, and so is Bonnie, In a final episode of the season Fiona doesn't want to give her liver to Frank, so he goes and sees his OLDEST daughter.
In season 5, the Gallaghers find out Ian is bipolar. Carl teams up with a gang of drug dealers and spends his time dealing small amounts of drugs for money. Carl is eventually given the task of transferring some heroin to Michigan. Carl seeks advice from Frank. Frank warns Carl that he may be easier caught than someone like Chucky. Carl agrees, and Frank gives him the money to cover the bus fare to Michigan. Carl, having earlier convinced Chucky to do things for him in exchange for protecting him at school, takes advantage of Chucky and straps pounds of heroin to his body using duct tape. Chucky isn't fully aware the bags contain drugs and agrees to accompany Carl on his trip to Michigan. The pair arrives at the bus station and as Chucky sits down, Carl spots a security guard with a detection dog and leaves Chucky behind. The dog begins to bark and Chucky is caught and arrested. Sammi later shows up at the Gallagher household and accuses Carl of forcing Chucky to assist him. Carl repeatedly denies her accusations and eventually, the police show up. Carl's siblings tell him to run, to which he bolts out the back door and down the street before getting caught by the police. Back at the house, Sammi admits she told the police it was Carl, and she also told Chucky to tell the police that Carl made him do it. Carl is held at the police station several hours for questioning. The next day, Sammi confronts Carl while he is being held and threatens him, to which he replies, "Bring it, bitch." Sammi attacks Carl and punches him repeatedly. In court, Chucky is sentenced 120 days in juvinile prison, or 60 on good behavior. When asked if Carl regrets his decisions, he simply admits he only regrets trusting Chucky with the job. The judge is appalled and tells him he will end up in juvie, and follows by asking if that's what he wants. Carl replies, "Yes, please" and humiliates the judge with a vulgar comment. She sentences him to the maximum time of one year in juvinile prison. Carl arrogantly tells his siblings he intends on making juvie his bitch and is taken away. Season 5 concludes with Carl and Chucky on the verge of a gang war.
In season 6, Carl is released from juvie a few months earlier than expected on good behavior. On his way home he befriended an older boy named Nick, who had been in juvie since he was a child but was released due to his eighteenth birthday. Carl allows Nick to stay at the Gallagher house, much to Fiona's dismay. Carl returns to school and brings Nick along with him. The two smuggle a backpack filled with a cache of weapons into the school and begin running a business selling them. Teachers quickly catch on including the school principal and a math teacher who approach Carl and to his surprise, ask to purchase guns of their own. Carl also takes interest in a classmate, Dominique.