Deborah 'Debbie' Gallagher is the fourth Gallagher sibling. Although she tries to act hard and comes off as a tough girl with a spunky attitude, she is known for being helpful with good intentions and has a caring heart. She's willing to bring her baby brother to school for show and tell when there's no one else to watch him, place a pillow under her father's head when he's passed out on the floor, and collect money for charity year round — only some of which she keeps for herself. She is one of the only Gallagher children to still be hopeful of Frank eventually behaving like a real dad, even with Frank's continued mishaps. He had inadvertently crushed a replica version of Abraham Lincoln's home made out of popsicle sticks, a school project Debbie was creating and she grabbed a large bag of soap and hit Frank with it. Frank was kicked out while Debbie wanted be alone in her room.
In the very beginning of the series Debbie Gallagher is described by Frank as being "sent from the gods." However, we soon learn that description is a bit of an exaggeration. Except not by much because she is extremely mature and grounded for her age. So much so that she has trouble relating and socializing with kids in her age group.
At one point during Season 3, Debbie expresses her desire to go swimming at the city pool. Frank tells her it's a bad idea because "at the city pool there are city kids." Apparently one summer, these city kids ripped Debbie's bathing suit to which she says they were just trying to make it into a bikini. Frank then tells her that if those kids will do that to her while running through the sprinklers in her own front yard, just imagine what they might do with an entire body of water at their disposal. Ultimately, Frank will not allow Debbie to go to the pool until she is able to hold her breath for ninety seconds, saying that if she can hold her breath that long that it will be long enough for her to get out of any trouble (should she ever find herself in it). Debbie soon is able to hold her breath for over one hundred seconds, ensuring her ability to swim at the city pool. However, the type of city kids she runs into are not what Frank was expecting, they come in the form of young pubescent girls with whom Debbie attends school. They tease her for her childish one-piece bathing suit, ask her if she's a lesbian, and tell her she should "grow some tits." The next time Debbie goes to the pool she sneaks a white bikini from Fiona's drawer and adds extra padding on top with some socks. She bravely goes for a dip in the pool, and she comes back to sit at her chair and read a book. However, this moment of bravery was short lived as someone put ketchup on the chair Debbie was sitting at, and the girls scream at her, "period! period!" and everyone laughs. A humiliated Debbie rushes home and locks herself in her room and sobs. Fiona tries to console her by telling her that,"nobody fucks with the Gallaghers." Debbie gets the courage to go back to the pool and this is where her ability to hold her breath pays off. She fills two bags with sand which she uses as a weight to hold her down. As one of the girls who humiliated her is about to get out of the pool, Debbie grabs onto her leg and holds her underwater until she passes out. She's quite happy with what she accomplished and runs back to the Gallagher house, screaming, "That's right! No one fucks with a Gallagher!"
In Season 4, having two friends who glamorize sexual promiscuity, Debbie meets Matt, a 20 year old at an arcade who she attempts to seduce only to have him reject her. Although upset, she agrees to only become friends. Later, she is confronted by Matty's apparent girlfriend who threatens her with a baseball bat to stay away from him. Matty breaks up with his girlfriend when she turns the whole school on Debbie and he asks her to remain friends. She accepts but is secretly more devoted than ever to get him back as her boyfriend. In season 5, Debbie invited Matty to a party where he got intoxicated after drinking Frank's booze and then coerced him to have sex with her.
In Season 5, Debbie launches in to the world of sex and relationships. Debbie has a party and when her friend Matt (who she always wanted to be more) passes out drunk she climbs in bed beside him and after seeing his boner puts a condom on it and has sex with him - which is technically rape but Matt won't do anything about it because it might back fire remember he is 21 at this stage. After Debbie gets into a fight with Holly and is saved by Derek she begins to show an interest in him. Debbie attends the gym that he goes to for boxing and keep fit classes and then he starts to hang out with her more and begin to like her in the same way, they begin a relationship together after Debbie creates a scene in front of the Gallagher household. Debbie meets Derek's family and she learns about this girl his parents look after who is a teen mum and intrigued by this, she also learns Derek's parents were childhood sweethearts and had a baby when they were not that much older than Derek and herself. Debbie then convinces Fiona that she should go on Birth Control while attending the clinic the nurse tells Debbie that if she is planning on having sex in the next few days she should still use a condom because the pill will not work until that long. That same day Debbie goes home and she is with Derek he goes to put on a condom before having sex she says "it's okay I'm on the pill" and they have sex. In the final episode of the season Debbie has an altercation with Fiona and she explains she wanted to get pregnant and she knows who she loves and she knows what she wants.