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. She finds out Matty has started to date a girl who works with him. She become jealous and tries to make the girl stay away from Matty by threatening her and putting a snake in her car. Later, she is confronted by Matty's apparent girlfriend who threatens her with a baseball bat to stay away from him. One day at school a boy in her class starts to show interest in her. He asks if they can dissect the frog together, she accepts but her friends who are interested in him want to work with him too. He shows her all the frog parts, Debbie is not grossed out but her friends are very disgusted. When he gets to the frogs heart he gives her it. Debbie is very flattered at this gesture. They hang out more that day at school and he even sketches a picture of her. He asks her if they can meet up after his basketball practice, she accepts. They then are making out and he takes off her shirt when suddenly a bunch of people pop up and laugh and take pictures of her. Matty's girlfriend is there and says that the boy that has seemed interested in Debbie is actually her step-brother and she paid him to do this. Debbie is shocked and majorly heartbroken. Matty finds out and breaks up with his girlfriend, he asks Debbie 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 says she is pregnant. She tells Fiona she wanted to get pregnant and she knows who she loves and she knows what she wants.
Fiona hates this so she tries to convince Debbie to get an abortion. Fiona finds out she is pregnant too. Debbie is excited and says that they can raise them together. Fiona tells her that they can instead get abortions together and have babies when they are both ready. Debbie is outraged and storms off. She later goes to the clinic with Fiona but doesn't get an abortion and lies to Fiona telling her that she got one. Fiona finds out that she lied when Derek's parents come to the house to discuss the baby. Debbie believes Derek will stay with her but then Debbie finds out that Derek moved to Florida to live with his Grandparent's because he doesn't want to be a dad yet. Debbie continuously tries to contact Derek but he disconnects his phone preventing that from happening. She still decides to keep the baby. Fiona does not approve of Debbie having this baby and tells her she will not support her when she gives birth. Debbie decides she has to find a place for her and her baby to live. With the help of Frank she becomes a live in nanny for the couple since the mother was dying of cancer. Encouraged by Frank, she tries to seduce the husband of the dying woman so they won't fire her, he rejects her advances. Frank takes the dying woman to The Alibi to give Debbie some time to seduce the husband. There Frank finds out that the dying woman used to be a lesbian. Frank tells Debbie this and suggests that she try to seduce the dying woman. Debbie dislikes the idea but still spends time with the dying woman. One night Debbie is in her room when someone enters and gets into bed with her, Debbie thinking this is the husband says "I was hoping you'd come" when turning to see its actually the dying woman. Later on the dying woman finds out she isn't dying anymore and the cancer is almost gone from her body. Frank tells Debbie she must start sleeping with her or else she won't have anywhere to live. Debbie tries to sleep with her but then stops before anything happens telling the woman that she is actually 15 not 18 and she isn't a lesbian.
On the episode Grandma Queenie to the Rescue, Debbie is shopping at a baby store when she meets a man who asks her opinion on a gift he is buying his sister. She tells him her opinion. He tells her thanks and says his name is Larry. They then meet again that same day and he asks her out for lunch. While they are deciding what to get he starts moving his foot against her leg she then says "You know I'm pregnant right? 7 months" He tells her he is fine with it. After they are done their date it seems as if they are going to kiss but then he kisses her baby bump instead, Debbie looks confused by his action. The next day Debbie is going to her baby class when she sees Larry she asks him "How'd you know I'd be here?". He tells he didn't and is there with somebody else while pointing to another pregnant woman. Debbie is very confused and he tells her that he has a baby bump fetish, where he is attracted to baby bumps on women. Debbie is disgusted telling him that "she can't believe she fell for him" he tells her they can still date. Debbie asks what happens after she gives birth. Larry responds saying that they won't date anymore but they can be friends. She also finds out that he his dating several other pregnant women but she is his only 7-month-er. She says he is a sick person and leaves the class.