Phillip Ronan Gallagher, as known as Lip, is the second-eldest Gallagher child and the most academic. He is a straight A student with a 4.6 GPA. While he possesses above average intelligence and excellent mechanical skills, he deliberately wastes all his time acting like the rest of the people in his town. He uses his intelligence to make money for his family, such as taking the SATs for other students (often scoring a perfect 2400) or tactically avoiding the cops when selling beer and marijuana from an ice cream truck during the summer. But despite his above average intelligence (a genius intelligence, almost), he has terrible judgement, thus is frequently shown smoking cigarettes, having unprotected sex with multiple women, and drinking copious amounts of alcohol in his free time.
Unlike his other siblings, Lip has a juvenile prison record; Ian has been his accomplice several times, but Lip always covers for him because Ian wants to enter West Point. Lip has a highly cynical personality and is unrestrained in his criticism of almost anything, which often gets him into trouble. He also has a history of unstable relationship: two years of off-again-on-again dysfunction with Karen Jackson, and then a little over a year with Mandy Milkovich which had her carrying most of the effort (and realizing that it was not going to last, since Lip was always hung up on Karen).
Lip often gets into fights, sometimes arranged for money, but generally does not care if he wins or loses. While he has street-fighting skills, he is not physically fit like Ian, and his provoking attitude gets him beaten up frequently. Lip is dedicated to and protective of his younger siblings, but his pessimistic worldview and penchant for always assuming that there is no outcome but the worst causes him to clash with Fiona's attempts to better their situation. All the same, he is grateful to his older sister for the years she spent raising all of them. Lip and Ian (who is later revealed to be their half-brother) are very close, and often give each other relationship or career advice (though on two occasions this resulted in fistfights).
Lip is extraordinarily intelligent for his age but can be extremely irresponsible. He is self destructive in his own way and has problems helping himself. Lip is very loyal to his family and protective of his younger siblings, often clashing with Frank because of this. His playful nature can be seen through his interactions with his siblings, particularly Liam.
Throughout the Seasons
Lip spent much of the second season believing that he had fathered a child with Karen Jackson, and continued to pursue her throughout her pregnancy despite her new marriage to Jody. Learning of Karen's plans to place the baby up for adoption triggered a chain of events that led to his expulsion from school and a fallout with sister Fiona, after which he left the Gallagher family home and bounced between Karen's, Kev and Vee's, and even the El before finding sanctuary at Steve/Jimmy's hotel room at the Chicago Ritz.
It was only during the episode Just Like the Pilgrims Intended (the penultimate episode of the season) did he learn that the baby was not his. During Fiona Interrupted, he finally leaves Karen and moves back to the Gallagher house after receiving some advice from Mandy, and Karen decides to leave home, despite Lip telling her she will regret the decision. Steve/Jimmy eventually kicks him out, so he moves in with Mandy for a while.
In May I Trim Your Hedges?, a child rapist was found living 3 blocks away. This news was learned after Debbie was watched by a masturbating man (who was immediately scorned and chased away by Fiona) on the public bus. At home after the incident, Lip and co. discovered a slew of sex offenders living in the neighborhood, with the closest living just three blocks away from the Gallagher home. Deciding to take matters into their own hands, Lip and Ian enlisted the help of Terry Milkovich, the Milkovich brothers, and several other residents in ridding the neighborhood of the offender. Upon arriving to the marked house, the gang confronted the offender and found he to be a she - a woman named Blake Collins. Unable to harm a woman, the group scattered. But Lip was still determined to make the his neighborhood a safe place again. Determined to expose Blake as an unchanged child rapist, Lip posed as a 15-year-old offering to do some lawn maintenance for Blake (hence the titular question - "Can I trim your hedges?"/"It's may I trim your hedges."). During his time undercover, he was exposed to Blake's sexual ways and took pictures with his phone for evidence.
Prior to the beginning of Season 3, Lip began a relationship with Mandy Milkovich. Their relationship was not without their rough patches, however. Mandy confronted Lip about being treated like a tool. After some sharp advice from Ian (who, along with Debbie, believed Lip had been moping over Karen for too long), Lip realized that perhaps Mandy wasn't so bad after all. During their relationship, Mandy did many good deeds for Lip and the Gallaghers, such as making food for the kids and offering to watch Liam. She even sending out college applications, though this was met with initial disapproval from Lip. After coming to realize that Mandy was the best for him, Lip sent Karen a final message, berating her for her selfishness.
In later episodes, Karen returned to the South Side after months of absence. Her reappearance prompted Lip to face her again; however, coming face-to-face with her made him realize that he was still in love with her (or at least, he still had some feelings for her). Karen's return upset Mandy, who was devastated upon learning of Lip and Karen's affair. If things weren't bad enough, Lip grew upset when he learned that Mandy had been applying to colleges for him without his knowledge or consent, to which Mandy reacted with disdain. They later reconcile when she demands a thank you for the effort she put into trying to give him a better life. After having wild sex in Lip's bedroom, Lip asks her why she applied for colleges so far away, to which Mandy replies that she wished he would get into somewhere far away and would take her with him; but ultimately she wanted him to get out of the South Side because she felt he deserved to do so. Her sentiment touched Lip, prompting Lip to choose Mandy over Karen.
Later, Lip confronted Karen and revealed his decision to choose Mandy over Karen because of Mandy's kind-heartedness and selflessness; because Mandy is good for him, and Karen is not. This upsets Karen, and after Lip leaves she calls Mandy to tell her she is going to have sex with Lip using a broken condom so she can get pregnant and trap him in the South Side with her, not Mandy (who she know Lip will drop once Karen announces her pregnancy). This, of course, angered Mandy, who then became determined to get rid of Karen in any way she could. Mandy put her plan into effect by sending Karen a text from Lip's phone - impersonating him in the process - which asked her to meet 'him' in the park. Karen naively went to the park, expecting Lip to have changed his mind, only to be run down while crossing the street. It was later revealed that Mandy had mowed down Karen, who was then admitted to the hospital and placed in a coma.
Lip was shaken by the incident. He then visited Karen at the hospital and, after seeing her in such a state, began wondering how she came to be in her coma. After learning from Sheila about a text he had apparently sent, he began investigating. He knew he hadn't sent the message, but who had? While on the bus headed home from the hospital, he decided it must have been Mandy who sent the text. He didn't confront her initially, but instead placed an embargo on sex with her at Kevin's insistence. Kevin told him how bad of an idea it would be to have sex with Mandy, saying that sex was how women controlled men. Kev's advice failed Lip, who called Kevin on his phone after an attempt Mandy made that succeeded (possibly raping him). He visited Karen once she woke up from her coma and was released into Sheila and Jody's custody. He learned that Karen has brain damage and cannot make new memories, which explains why when Lip told her that Mandy had hit her, she did not remember when Lip asked her who hit her immediately after telling her.
Angry at Mandy's actions, Lip crashed Micky's wedding and confronted Mandy about her role in crippling Karen and causing her brain damage. He angrily broke up with her on the spot, but she replied with indifference and revealed that she had given a blowjob to a man named Kenyatta in the bathroom to make Lip jealous. Lip left the reception with a drunken Ian, who threw up on the way home.
By the end of the season, Lip was ready to graduate. He thought very little of this milestone despite his sister Fiona's insistence of its importance in the Gallagher family - he would be the first Gallagher to get a high school diploma. After claiming his diploma from Mr Healey at the school office, he met with Frank, with whom he had a steak dinner and went ice skating. Together, they got hammered, resulting in Lip throwing up on the ice. Frank soon followed, only to projectile vomit blood instead before collapsing on the rink, Lip and Frank then go to the hospital, and Lip spends time huddled around a toilet in Frank's room. At the hospital Fiona demanded that Lip sleep off his drunkenness. Lip left for home soon after. Days later, Lip visited Mandy. Calm and civil despite how things ended, Lip informed Mandy of his plans to enroll in college and thanked her for what she did (applying for him). Before he left, Mandy asked if she wanted to go. Raising his hand, as if he didn't know himself, Mandy watched as he walked away from her and began his new adventure as a college student - the first in his family (and likely the last for a while).
Lip began Season 4 as a college student struggling to adjust to the new adventure in his life. He began working a menial job in the cafeteria as a busboy, and struggled to keep up with the workload because of his lesser means (not having a computer for class like the other students, for example). To make things worse, he began receiving terrible grades in his classes, such as receiving a D on a paper for Byron's Childe Harold's Pilgrimage for a literature class. Despite everything, Lip made a friend in his roommate, Ron, who was very nice and allowed Lip to use his computer whenever he needed. Ron's girlfriend Amanda, however, was not as welcoming.
After Liam's hospitalization and Fiona's imprisonment for child endangerment, Lip's struggles grew when he was forced to take over as the head of the family. He often had to take Liam to school with him, but he received help from Amanda during this time. Amanda would watch Liam whenever Lip had class. Lip began bringing home food from his cafeteria job to help feed his family after Fiona was placed on house arrest.
Lip began getting his act together towards the end of the season to help keep his scholarship. He also began a relationship with Amanda, who was revealed to have annoyed Ron with her constant talking. Ron was glad to get rid of her in the end.
Lip came home during summer after his first year, and got a job with Tommy's construction company. Ian told him that Mandy was planning to move to Indiana, and asked for Lip's help in stopping her. At Mandy's house, Lip and Mandy had sex; and during sex, Mandy told him that she loved him, to which Lip gave no reply. Mandy left with Kenyatta the next day.
In the fourth episode, Lip flew out to Miami to stay with Amanda and her family. There he and Amanda had sex for the first time. Her father seemed dismayed with his arrival, going so far as to locking his liquor cabinet. He and Amanda later had lunch with her parents, during which he and Amanda's father bonded over her father's new projects. He later smoked pot with Amanda's father in private, and during their private talk Lip was offered a paid internship.
In later episodes, Lip accompanied Mickey Milkovich in shooting up a brand new coffee shop to be opened in the town as a part of the gentrification of the South Side. Lip was surprised when Mickey and the other guys pulled out some machine guns from the truck, and refused to take a part in the shooting. Mickey called him a "college bitch" when he realized that Lip hadn't shot the store, and when sirens could be heard, Lip bolted and jumped into a dumpster in a nearby alleyway.
After the incident at the coffee shop, Lip decided to return to school early to help set up for the new school year. For his second year, Lip applied to become a Resident Adviser, and was thus given his own private room. Sometime after returning to school, Lip began a relationship with Helene Wallace/Runyon, a professor at the university.
Lip was also the first person to learn about Fiona's spontaneous marriage to Gus Pfender, information he learned during his conversation with Fiona about going back to school early.