Liam Gallagher was originally the youngest of the Gallagher children, until the birth of his younger sister Stella in Episode 516. He was also a minor character until Series 5, when he took a more central role in the main storylines. He apparently suffered from fits brought on by Epilepsy when he was younger, according to Frank in Series 1's Opening Credits and in Episode 7 of Series 7 (although this was never presented onscreen). It was once implied that he used to wet the bed, though he claims that it was only to help put out a fire, something he was obsessed with from Series 4-5 (on the other hand, it could have been due to an epileptic seizure). Throughout the series, Liam was practically raised by his older sister's, Fiona and Debbie, as Frank is lazy and Monica inept- even when she is around. In later episodes, Liam showed considerable intelligence and resourcefulness like sister Debbie and older brother Lip, and has also been an outspoken atheist since Series 3 when he was just five years old. In Series 5, Liam was shown to be an adept car thief, schooling Shane and Micky Maguire who were clueless at it, and would blackmail them whe they refused to pay him. In Series 6, it was revealed that Liam is a talented musician at the Balalaika and Guitar, and also fluent in Sign Language (upon first meeting Danny). Given his academic performance, the school determined Liam to be an exceptionally gifted pupil during episode 10 of Series 6, which led to him being awarded a special scholarship. During an interview at a local university, he initially failed to make a good impression due to nervousness. But that changed when his father, Frank, attempted to steal the "Joy of Sex" manual from the university library; Liam plucked up the courage to tell the panel of education directors why he deserved the scholarship in an impassioned speech, while Debbie looked on proudly.
Liam was also one of the few people in Chatsworth who was aware of Micky Maguire's homosexuality. He sarcastically told Micky that he figured it out through his "amazing powers of deduction", despite Micky's stereotypically gay mannerisms and aesthetic preferences.
During Episode 13 Series 6, Liam and his sister Stella were taken into foster care. A social-worker named Josie was investigating the household, when Debbie was forced to confess that Monica had left several months earlier. During the meeting, Yvonne dragged in a drunken Frank, complaining of his loitering. The next day, Frank had to clean up his appearance for Josie. She remained sceptical, but talked Debbie, whom she rightly surmised was looking after the two younger children, into going on a night out. The next day, Debbie returned home to find a police car parked outside with Josie, Liam and Stella inside. Josie had arrived earlier to find Frank passed out on the floor, and all others absent but for a ten year old Liam looking after Stella. Despite being forbidden to contact his family, Liam got round that by means of Xbox Online and achieved contact with Chesney, who in turn informed Debbie. Later, the Gallaghers convince Yvonne, a registered foster-parent, to accept Liam and Stella into her care so they could remain close to their older siblings living next door.
Liam has been noted for his Atheism since the age of five during Series 3. Debbie's influence as her younger brother's role-model largely informed his opinions. During Series 6, Liam seemed to reconsider his position on the matter when he told his sister that he didn't want the clothes that she and Maxine stole from various shops, because they brought "bad karma". In the same episode, Monica tricked two Jehovah's Witnesses into babysitting Liam and Stella, while they discussed religion with her son, telling them that her son's soul was "up for grabs" (this was so she could go to the pub). Liam ran circles around the two men, pointing out flaws in their belief system. Afterwards, Liam confided in Monica that "they may not know it, but I'm wrong and they're right". During the trip to Blackpool for commercial photography, it was implied that Liam thought of an escaped mental patient hiding out in a gas station as resembling Jesus. Liam tried to conceal his t-shirt which had the words "God is Dead" printed on it, while the old man tapped a badge on his chest simply saying "No He's Not". Liam ended up giving the old man an envelope of money later on. In Series 7 however, Liam seemed to have re-affirmed his position on religion during a conversation with Libby, after she sent her mother Patty to Lourdes, where Liam was sceptical about the existence of deities or an afterlife. Liam also showed himself knowledgeable about the Bible and various other religion's holy books when he informed Kelly Ball about areas which condoned prostitution and later looked for loop holes in people's chosen religions in exchange for money.
In the earlier Series, he also collects Dual Master Trading cards in one scene in the kichen at the beginning of an episode (Episode 304?).
Series 7 of Shameless saw Liam take on the responsibility of running the Gallagher household in the wake of his sister Debbie's departure from Chatsworth. The responsibility got on top of him as it was making him late for school, due to receiving no help from either his father Frank or his brother Carl. When the household fell behind on their HP payments in episode 6, an unhelpful Frank blamed his eleven year old son, calling him a "massive disappointment". Liam gambled at Joe's shop (formerly Yvonne Karib's shop), showing formidable skills at poker for much of the game, only to lose to Joe's experienced hand. Joe continued to raise the interest on the money Liam owed until it rose to £1500. Liam later met his father at The Jockey, telling him in vain that he should be helping his family and doing something about their debt. When Liam started to get angry and told Frank "I don't know what to do, and its all your fault", a drunken Frank headbutted him, to the shock of everyone present. Carl, who was present, attacked Frank in a rage. The following day, Liam phoned Joe, asking him if he died, would his debt be void. Joe hurriedly found Frank, Ian and Carl and brought them to a building where Liam was threatening to jump. Frank apologized for his behaviour and made a string of excuses, which didn't impress Liam while Joe promised to help him clear his debt. When Frank promised to be more supportive, Liam told him that he never kept a promise and seemingly jumped off the building, only to intentionally drop to a lower balcony, much to their relief. Liam held both Joe and Frank to their promises.
Liam was attending a local grammar school where he met a girl named Holly during his music lessons who invited him to dinner with her family. They were upper-middle class professionals, so Liam lied about his background out of embarrassment. He claimed his mother was a nurse, and his father was a deceased heart surgeon. Later, Liam was day-dreaming about Holly during music class, causing him to fall off the stage, sustaining mild injury. The school alerted Frank to the situation, and he arrived to see his son refusing a lift home from Holly's father. Liam's carefully constructed back story fell apart when Holly and her father learned that Frank was not only still alive, but is utterly different to how Liam described him. Holly's father was about to leave with his daughter but told an ashamed Liam that he didn't blame him for lying, as he would have done the same. Frank voiced his objection, but got told to shut up by Holly, who described him as behaving like a "rancid, smelly old tramp". However this turned Liam against Holly, causing him to storm out of the building without saying a word, to accompany Frank home.
Liam was initially reluctant to accept Libby Croker, Frank's new girlfriend, into the family, fully aware that Frank always found away to ruin his relationships and cause them to leave. He responded to Libby coldly at first and later with open hostility after receiving a letter personally addressed to him by Debbie to say that she was being posted to Afghanistan. Tortured with worry, he found solace in the company of an elderly WWII veteran fighter pilot named Reggie, who his school had assigned to interview on his history project. When his family found out that he had been spending time after school with the old man, Carl and Libby were concerned that Reggie may be a paedophile and demanded that Liam cease visiting him. Reggie was in fact sickly and close to death and at one point was so overwhelmed with the knowledge of his imminent death that he hugged Liam, who was helping him out of his chair. Carl and Libby's concerns had troubled Liam, who mis-read Reggie's reaction, pushing him off angrily and ran out of the house in horror. Liam later returned to a dying Reggie's bedside when he realised his earlier irrationality. Frank, Carl and Libby followed him to the house, suspecting incorrectly that Reggie was molesting Liam, only find him grieving at Reggie's side. Overwhelmed with grief, Liam cried into Libby's arms.
Liam was later targeted by bullies from his school when one of them spotted Liam wearing a dress. One of the bullies, a fourteen year old boy named Dean, was found dancing on his own in an alleyway by Liam the following day. Dean accused Liam of "perving" on him and chased him down. He then suddenly tried to kiss Liam, who recoiled. Dean was genuinely shocked by Liam's reaction but tried to maintain his facade among his friends by dying Liam's hair green and smashing his guitar. Carl and Libby were later fussing over Liam at home when Kelly and Shane burst in, demanding to know who did this, which lead to an argument between Libby and Kelly. Libby later fixed the damage done to the guitar and at Frank's insistence, Liam started acting thankful toward Libby. Liam was later being hounded through the streets by the bullies, but this time Dean helped Liam escape on the back of his bike. During the talent night at The Jockey, Liam's confidence on stage faltered when he was being heckled by bullies, but Dean, who was himself initially afraid of performing in public, demonstrated his moves before the regulars. This rubbed off on Liam, who picked up the pace on his acoustic guitar and both received a positive reaction from the crowd. Liam and Dean became close friends afterward.
During Series 8 Monica attempted to ruin Frank and Libby's marriage by having Frank committed to a Psychiatric Hospital, accomplished through her lover being a member of staff. She was also using Frank's absence as a way to take Liam and Stella away to live with her, her reasoning was that they should be with a parent but it was revealed she only wanted them to get a bigger house from the council. But following the break down of her plan Liam announced he would move in with her if she allowed Stella to stay with Frank and Libby, telling Monica that he'd make her life difficult for her otherwise. In the fifth episode when Frank and Libby were both present at their wedding (which was never completed due to the kidnapped priest leaving), Carl arrived with his Best Man's speech, which turned out to be a personal message from Liam, saying that he's decided to stay with his mother, claiming that he wanted to "fix her".
- Liam shares a birthday with his father Frank. He turned 11 in Episode 1 of Series 7 while Frank turned 50, a shared birthday party was thrown for them.
- Liam had consistantly slept with the same animal-print duvet throughout his appearance in the series. It remains in the Gallagher home after he leaves, being used by others such as Aiden Croker, Carl and Lip Gallagher.
- It has been hinted twice that Liam practices Karate. Once in Episode 6 of Series 3 when Ian forgot to collect him from his class (held at a community centre, where Liam witnessed an Alcoholic's Anonymous meeting), and in Episode 4 of Series 6 when he was briefly seen in a Karate uniform.
- Liam briefly demonstrated some physical toughness when he headbutted his father, Frank, in Episode 701.
- Liam is said to suffer from Epilepsy, although he has never exhibited any seizures. In Episode 7 of Series 7, Frank claimed that Liam's epilepsy used to be more rampant, saying that he was like "Shaking Stevens" before doubling his medication.
- In Episode 501, Frank claims that he used to give Liam, between the ages of three and four, a couple of downers, to get him to "shut the fuck up".
- In the Virtual Tour of Chatsworth, Frank admits in his Curriculum Vitae that he named his youngest son after Oasis singer Liam Gallagher, after briefly working as a roadie for the band in 1994.
- Only two characters in the series, Billy Wilson (Episode 106) and Lillian Tyler (Episode 301), have made mention of Liam sharing his first and last name with the former Oasis singer.
- In Episode 908, Frank was recounting the names of his children when he was bemoaning the disappearance of his trademark parka. As well as Ian, Frank struggled to remember Liam's name, calling him the "curly-haired one", until it was blurted out loud by Terry, one of the Jockey regulars, even though he was in a drunken stupour.
- It was somewhat implied by Monica in Episode 13 of Series 11 that Liam is no longer living with his mother, when she expressed her wish to see her seven children. Although there was no explicit mention of him in the episode.
- However, he was mentioned sporadically in Episode 14 of Series 11 by Lip, whom vaguely stated that Liam "had his act together". Lip made a passing remark about Buddhism, which Liam may be involved in.
- In the same episode, Lip arrived at the Gallagher house, worried that Liam may have died, after being contacted that a funeral was being held. But the deceased sibling in question turned out to be Monica's aborted embryo, whom she curiously named "William".
- Liam may have been around fifteen or sixteen by the time of Series 11.
- In the Tour of Chatsworth, Liam may have glued the enclycopedia pages on the bedroom walls, as they did not appear prior to Series 7.