Courtney Day DiLaurentis was the identical twin sister of Alison. It is revealed in Ali's Pretty Little Lies that she actually wasn't mentally unstable like everyone believed, but Alison was. She was more popular in school and Alison was jealous, so she framed Courtney for being mentally ill.
Courtney spent 3 years in Radley and when that closed down, she was going to be sent to the Preserve, but she tricked their mother into taking Alison instead as revenge for Ali ruining her life. A year and a half later, Alison shoved her into a deep hole in the ground that was under construction for a gazebo they were building, killing her. Three years later, her body was found in the same hole when the St. Germains were tearing down the gazebo.
Before "Ali's Pretty Little Lies" it was believed that Courtney was insane. However, it was revealed she was just misunderstood and framed by Alison. Courtney was the "good" twin, sweet and sympathetic. When she took over her sister's life, her personality morphed into Ali's. She became calculating, cruel and secretive, turning into a manipulative bitch as time progressed.
Before the switch, Alison was slightly prettier that Courtney, but people were drawn to Courtney more as a result of her more inviting personality. Alison noticed that people saw her as bossy and mean, so she took matters into her own hands. People began to be afraid of Courtney as Ali's lies grew, eventually swapping with Courtney for her "rightful" place in the spotlight.
Courtney's hair was long, wavy, and honey-blonde. She had deep cerulean eyes. Like her sister, her face was heart-shaped, and her lips bowed. The two were almost exactly identical. However there were small differences; her eyes were thinner and less blue than Ali's, and her hair was slightly lighter, longer and less wavy. Ali's waist was slightly thinner and her chest slightly bigger. Her face was also more heart shaped than Ali's. Ali thinks of herself as the more beautiful twin and even Courtney once said that Ali always looked a little better than her.
Despite their initial friendship, Courtney was tormented by her twin. Alison was always jealous of her more popular sister and forced Courtney to impersonate her, at the same time torturing her to the point where her sister got into a physical fight with her, attempting to strangle her. When doctors were called by the girls' parents, Courtney panicked and revealed that Alison was threatening her and forcing her to impersonate. An innocent-acting Alison claimed it was not true, and, when the final decision was up to her, decided that her sister had to go, which was allegedly for the safety of everyone. In the outcome, Ali's master plan worked: Courtney was diagnosed with paranoid schizophrenia.
The Day-DiLaurentis family moved to Rosewood (changing their last name to simply "Dilaurentis"), and Courtney was placed at the Radley, a mental institute. She occasionally came home on the weekends, but was forced to stay in the house, as the DiLaurentis family pretended they only had two children - Jason and Alison. She stayed there for almost three years until the Radley was closed down. Her parents planned to send her to the Preserve at Addison-Stevens in Delaware after Alison falsely claimed Courtney threatened her, but she did not want to start over.
One weekend, Courtney was staying at home while her parents were planning to switch her between institutes, and she was having yet another fight with Alison. It was the same weekend the Time Capsule competition at Rosewood Day School was announced, and everyone wanted to find or steal a piece of the flag. Aria, Spencer, Hanna and Emily each wanted to steal Alison's piece for their own reasons, but when they arrived in the DiLaurentises' backyard, they noticed each other and came out to confront each other. Courtney noticed this from the house and alerted Alison, who only brushed it off, revealed her lie and started teasing her sister again. The Liars heard Alison maliciously yelling at her sister, although it was believed Courtney was the one yelling. Courtney had stolen Alison's "A-is-for-Ali" ring and she pretended she was her sister, lamenting about her flag already being stolen and discussing what she drew on it. Her mother Jessica, believing she was the real Ali, came out to tell her that she was leaving. Courtney barely acknowledged her and kept up the chatter to see if her mother would really be fooled into taking Ali in her place. Jessica hesitated briefly, but she then left with Ali in tow.
Courtney was immensely relieved that her plan worked; she was now Alison DiLaurentis. She had to figure out her sister's life quickly and so she read Alison's diary. She then realized she could not maintain Ali's friendship with Naomi Zeigler and Riley Wolfe, because there were things she did not understand about them. Instead, she deliberately got in a 'fight' with Naomi and Riley and dumped them, making them uncool. She set her eyes on the four girls she talked to in her backyard, Spencer, Hanna, Aria, and Emily. The girls were shocked but flattered that the most popular girl at Rosewood Day wanted them as her friends. Throughout sixth grade, the five most popular girls in Rosewood had a great time together. She had always considered them her best friends, even getting jealous when they were not around her.
Their lives suddenly grew more complicated when Courtney decided to get revenge on her neighbor, Toby Cavanaugh for spying on the girls while they were changing. She intended to launch one of his fireworks near his tree house, but she saw him molesting his stepsister Jenna Cavanaugh. She was so shocked that the firework launched out of her hand and directly toward the tree house. As a result, Jenna was blinded and Toby took the blame, even though he saw Courtney and intended to tell the police. However, Courtney threatened to tell everyone about his sexual assault of Jenna, so Toby in the end confessed to lighting the firework and he was sent off to boarding school in Maine.
The girls were horrified by the event and named it "The Jenna Thing." After the accident, Courtney made friendship bracelets for the girls and made them promise to never tell anyone what really happened. Three years later, Aria confesses to Jenna that they were involved in the accident, but Jenna tells Aria that she and Courtney had bonded over "sibling issues" and they had planned the whole thing to get rid of Toby. And even though it did not go quite to plan and resulted in Jenna losing her sight, she could not be happier that Toby was finally gone.
Supposedly, one of Courtney's biggest secrets was her alleged affair with Ian Thomas, a high school senior who was dating Spencer's sister Melissa and whom Spencer also liked. In fact, it was later revealed that Courtney did not actually like Ian and was creeped out by the age difference between them; however, she pretended to like him in order to convince him to kiss Spencer. Initially, this was both to make Spencer happy and to ensure that the two remained friends. She promised to kiss Ian herself if Ian kissed Spencer.
Unfortunately, right as Ian kissed her, Courtney's new boyfriend, Nick Maxwell, caught them, leading Nick to dump her. Courtney's misery over the breakup was a major source of much of Courtney's nasty behavior towards her friends in her last few days. To celebrate the end of seventh grade, Courtney and the others decided to have a sleepover to kick-off the summer and on the evening of the last day of school, everyone gathered in Spencer's barn apartment. However, that same weekend, Alison was home from the Preserve and was furious with what Courtney had done, leading Ali and an unknown person to killing her.
While the other girls were generally happy, she made teasing statements to them which made everything tense between them all. She told everyone about how she knew how to hypnotize a person and insisted on trying it out on them (which, unbeknownst to the Liars, was to keep them from noticing her sister spying on them, and also to be sure that they stayed loyal to her forever, in case Alison tried to switch their lives back). It took some convincing, but they eventually agreed. Before she could get to Spencer - the last one not yet hypnotized - Spencer had jumped up and refused to do it. She opened her window blinds to let some light in, but Courtney noticed her twin hiding right by the window with a camera. She ignored her (though was secretly terrified that her friends would see her) and argued with Spencer before storming out. Spencer followed her to the edge of the Hastings property and they continued their argument. After Spencer shoved her lightly to the ground, Courtney, in anger, dropped the (false) bombshell that she was seeing Ian and another (true) bombshell that she convinced him to kiss Spencer. As she continued to taunt her friend, Spencer shoved her into the stone wall of the Hastings property. Courtney recovered quickly but noticed someone up ahead in her own backyard. Knowing that it was Alison, she ran off and ignored Spencer, who would be the last Liar to see her alive. When Courtney runs into the woods, Alison appeared and began threatening and taunting Courtney. Then the two girls began wrestling on the ground. Courtney said that all she wanted was for the two of them to be sisters and Ali says that she wants to kill Courtney, but she can't. At that moment Ian Thomas came, then Alison pushed her into the hole for the gazebo. She recognized both voices, her sister's and the unknown person's (it is not said in the book who the unknown person is). Courtney makes speculations as to who the unknown person is, but the identity of this person is not revealed.
Alison's plan A was to make Courtney switch back places with her, but when that didn't work, she switched to plan B. Plan B was to kill Courtney and replace her. Alison and her accomplice throw Courtney into the hole and pile a lot of dirt on Courtney's body, unaware that she was still alive. Alison slips back inside, hoping that her parents won't see her, but they do. At first they call her out as Courtney, but she argues it and they let her go upstairs. Alison thinks she has won, but in the morning her mother questions her about the where-abouts of Courtney-as-Ali and notices that Alison is not wearing the Jenna thing bracelet or the A-for-Ali ring. Jessica DiLaurentis freaks out thinking that Ali-as-Courtney has done something horrible to her twin. Alison is taken back to the Preserve at Addison-Stevens. As they drive away, Ali sees Courtney's best friends and blames them for her problems. In her mind, if they hadn't been in the backyard that day, she wouldn't have been mistaken as Courtney and taken to the Preserve at Addison-Stevens. She plans to get revenge on them (thus the idea of "A" is born).
The media stakes out the DiLaurentis mansion and jumps on the story of a beautiful rich girl who is "missing." A campaign to find "Alison" was soon started by her family. Three years later, the St. Germains moved into the home and were planning to put in a tennis court where the unfinished gazebo stood. When the construction team excavated the concrete, Mrs. St. Germain (Maya's mom) saw Courtney's body and screamed. The police swarmed the property and reopened the investigation as a murder case. Courtney was buried under Alison's name.
Though Courtney had clear control issues and necessity to be needed with her friends (which likely stemmed from having her stolen away and being unjustly forced to live in a mental institution for three years), she truly cared for them and (due to these insecurities) was very jealous when they seemed interested in hanging out with people other than her (though, hypocritically, she would do the same thing—mostly to make her friends feel the need to work harder to keep her, and thus, ensuring that they would not want to leave her). Courtney was initially a very good friend to the Liars, leading the five girls to have an easy and happy friendship at first; however, Courtney's crumbling home life, her friends' secrets and unwillingness to confide in her, the knowledge that her sister would be coming home from the Preserve soon, and, later, her breakup with Nick (and her inability to talk about any of her big problems to her friends) all placed a great deal of stress on Courtney, leading her to become much nastier and meaner to her friends in her last few weeks. Courtney also seemed to have become more vain as a result of her popularity as Alison, and, as a result of acting like a bitch for so long, was starting to actually become one by the time she died. However, Courtney was not a truly mean person (and acted meaner than she wanted to at times in order to make her charade as Alison more plausible), and always loved her friends. Contrary to Alison's belief that Courtney only befriended the Liars to get revenge on her (since Spencer, Hanna, Aria, and Emily were girls that the real Ali would never have befriended), Courtney genuinely liked her friends for who they were; when Alison insulted them, Courtney protectively stated that there was nothing wrong with her friends and that "they're very sweet." Unfortunately, Courtney never got a chance to tell her friends what she truly thought of them before she died, leading them (and the rest of the students at Rosewood Day) to think that "Ali" was a much bigger bitch than she really was.
After the initial friendship and "Indescribable Twin Connection", Ali started to be jealous of Courtney being more popular than her so much that she tormented her and framed her into being insane. The two sisters hated each other, and Alison was constantly teasing Courtney, treating her with superiority. When she was forced to take Courtney's place at the new mental institute, she was livid. She had a perfect life in Rosewood and Courtney successfully co-opted it, all because of Spencer, Emily, Hanna, and Aria's unexpected appearances in the yard. They provided the perfect distraction for Courtney and Ali hated them for it. Though Ali despised Courtney, Courtney was almost able to talk Ali out of killing her by stating that she just wanted them to be sisters again. However, Alison's accomplice in the crime (Nick Maxwell) showed up, causing Ali's hatred for her sister to win over. Ali killed Courtney, and, though she seemed shocked at first by what she'd done, she later convinced herself that Courtney deserved it (which was not true), and did not regret killing her. She planned to take her life back afterward, but her parents went crazy when they found out that she did something horrible to her twin, and she was sent straight back to the Preserve.
Spencer and Courtney had a complicated relationship. They were close friends and shared many interests, but they often got into fights the most. Being raised by no nonsense lawyers, Spencer was the only one that was tough enough to stand up to Courtney. Courtney often teased Spencer about not making the JV hockey team and having a crush on Ian. She was also secretly resentful that Spencer was unwilling to tell her she had a crush on Ian. On the night of her death, Courtney taunted Spencer for always wanting what she had and never being able to be as close to Ian as she (supposedly) was. Spencer's last interaction with Courtney thus ended on a negative note. After their fight, Spencer claimed to see two blondes in the woods behind Courtney's backyard, but thought nothing of it.
Besides Spencer, Aria was the only other friend who wasn't totally enchanted by Courtney. In fact, she felt a keen resentment toward her for constantly bringing up her father's affair with a college student. In reality, Courtney was originally trying to make Aria feel better and be there for her; however, she became resentful that Aria didn't want to confide in Courtney about her feelings, leading her to tease Aria about it. She was there when Aria discovered them making out (and, in fact, made sure that Aria discovered this, having already discovered it herself). In the last several weeks of Courtney's life, Aria avoided her as much as possible, unable to deal with her teasing.
Hanna idolized Courtney and wanted to emulate her. Of the four girls, she was the most surprised and flattered that "Ali" wanted to be friends with her. She was overweight due to her stress-based overeating after her parents' divorce, and Courtney's jokes about her weight made her feel worse. Hanna never fought back and sometimes felt like she deserved the teasing, believing she was worthless. Courtney would tell her lies such as fidgeting would make her skinnier (though she was actually likely trying to do this to make Hanna feel better), and Hanna, usually believing her, never stood still after that. Hanna was usually able to sense when Courtney was manipulating her, faster than the other liars, though, Courtney gave her credit for that. In Ali's Pretty Little Lies, Courtney said that Hanna wasn't that bad of a dork, and that she always knew what accessories and latest-season clothes to wear, and someday, if "Alison" ever gave up the throne to popularity, and Hanna did herself over, that Hanna would make a decent queen bee herself.
Emily also adored Courtney, but not for the same reasons as Hanna. She had strong feelings for her friend and expressed them by being friendly to her one day. Unbeknownst to Emily, Courtney already knew about Emily's crush on her, having read it in Emily's notebook, but did not acknowledge it until Emily kissed her. Courtney was initially shocked but soon found it a source of amusement. She often pointed out things connected to gay people, such as the gay parade, and asked Emily what she thought. Emily was embarrassed by her crush's teasing, but she maintained a sense of overprotective feelings. In fact, Emily was Courtney's favorite, and Courtney felt more guilty when she teased Emily than she did about anyone else she teased. Courtney jokingly called her "Killer" because she defended her like a possessive Rottweiler. It is revealed in Ali's Pretty Little Lies, that Emily was probably her favorite, and she loved it when she complimented her, even when Emily said "I can't believe I know you" it meant a lot to Courtney, even if she didn't show it.
Ian was quite philanderous and liked flirting with other girls, especially Alison DiLaurentis. Both the real Ali and Courtney were creeped out by him; however, Courtney, knowing that Spencer liked him and wanting to both help Spencer and earn her gratitude, flirted back with Ian, promising to kiss him if he would kiss Spencer (though, due to her relationship with Nick, Courtney never intended to hold up her end of the bargain). Courtney, disgusted by Ian's willingness to cheat on his girlfriend with her own sister, later demanded that Ian break up with Melissa before she would kiss him (though this never ended up happening). This promise of a kiss ended up leading to the end of Courtney and Nick's relationship; when Ian forcibly kissed Courtney, Nick caught them and broke up with her on the spot, leading Courtney to be miserable. Due to Courtney's flirting with Ian, Melissa began to (falsely) suspect that Ian and "Ali" were having an affair. Courtney later falsely claimed to Spencer during a fight that she and Ian were having an affair, and revealed that she had told Ian to kiss Spencer, leading Spencer to shove her hard against the wall. Courtney wanted to meet with Ian the night of the sleepover. Unfortunately for her, Ali got to her before Ian did. He actually saw the two arguing (as did Spencer) but he was drunk, and he was only able to tell that the person with Courtney was blond. It is unknown if Ian is the unknown person that helped the real Alison kill Courtney. It is doubtful though, since he was unaware that Alison-as-Courtney had a twin. If he had been aware of the twins, he would have brought it up when he was arrested for the murder of Alison. It is more likely that Ian just had a perverted and obsessive crush on Alison and that he stumbled drunkly back to the Hasting's barn after seeing the two girls fighting.
Before the switch happened, during Courtney's visit home, she and Jenna and Alison played Barbies in the backyard one afternoon. Shortly after the switch, Alison came home for a weekend and when Courtney wasn't watching, Alison escaped the house and told Jenna about the switch. Courtney tormented Jenna in school for an unknown reason, but Jenna never told the secret to anyone. Shortly before "The Jenna Thing" happened, Courtney saw Jenna in tears. Courtney suddenly wanted to atone for her nastiness and asked what was wrong. When Jenna told her that Toby was molesting her, Courtney decided to help her. Courtney helped to launch a firework but ended up missing and hitting Jenna in the face, leaving Jenna blind. Though a tragedy came of the firework, Toby was sent away after Courtney threatened that if he didn't fess up, she would spill Jenna's secret.( Toby was molesting Jenna). He did and was sent away.
One day when Courtney was in the Rosewood Day Parking Lot waiting for Jason to pick her up, she stumbled upon Nick Maxwell. Once they begun talking, he asked her if she remembers him, to which Courtney said "Refresh my mind," since she did not know who Nick was and what role he played in Alison's life. He told her that she (the Real Alison) wrote on a cabin wall at their old camp, Camp Ravenswood, that he was a good kisser, even though she never kissed him. They begin flirting, and Courtney slips him her number, realizing that Nick must be an old crush of Alison's.
Along the course of the book, the two start dating (and Courtney kept it private because she didn't want her friends to know about him yet), as Courtney finds him attractive just like Alison most likely did. They go on their first date at a Paint-Ball place, and they both win. A little while after that, they go to the mall and ride on a carousel for their second date, until Courtney sees her mother with an unknown man, to which she ran out of the mall. Later, Courtney plans a party that lots of kids end up disappearing from and leaves one dead, and she asks Nick if he will come, but he said he had work that night and he could not get the night off.
For their third date, Nick took Courtney to a picnic at Floating Man's Quarry. As they are eating on a picnic blanket, Courtney starts to get upset, and confides in Nick. She tells him about a secret twin that she has, and how her twin was unstable, but coming back home to live with them for a test trial. She also tells him about how she "doesn't know who her friends are anymore". Nick reassured her that everything would be fine, and that he would always be there for her. Once they were done talking about her twin, Courtney went into the bathroom to freshen up. Once she gets in there, a drunk Ian Thomas comes in, and pressures her for the kiss that Courtney promised she'd give him if he kissed Spencer. As Courtney calls the deal off because she was already dating someone, Ian pushes her up against the wall and kisses her. Their hands entwine, and they kiss for a few seconds until Nick comes into the bathroom to find "Alison". Nick gets upset and yells at both Ian and "Alison", then runs out of the bathroom. As Nick is running, Alison tells him that she has to explain what happend and why it happened, but before she could get to him, he jumps off the cliff into the waves below. That is the last time Courtney saw him before she died.
Courtney was devastated about their breakup, that she texted him lots of times and tried to call him too, but he wouldn't answer her, and he blocked her from his IM list.
It is later revealed in Deadly that Nick was the "Third A" and was Alison's accomplice. He helped Ali the night she killed Courtney.
Courtney's death scene is depicted in Ali's Pretty Little Lies. She was thought to be killed by Real Ali when she was pushed into the pit. Little did Ali know, Courtney was not dead. Nick and Real Ali covered her alive body with dirt and ran off. Her last word was, "Why?" (wondering why Nick did this to her).