<P> Actress Sasha Pieterse was cast as Alison DiLaurentis in December 2009 . Alison was the queen bee of her school and the most popular girl before her disappearance . Although she cared for her friends, she enjoyed to use their strongest secrets against them to that they could, according to her, stay together . Charming and manipulative, Ali was skilled at finding ways to get back at everyone, and many people hated her . After the reveal of Alison's fate in the ending of season 4, Pieterse confirmed that DiLaurentis' personality had drastically changed; she had to "figure out who she is . She's been hiding for so long, and she's a victim now ." "Alison's a totally different person, so to see the way they treat her is going to be very interesting and to see the way that she gets affected by it is also going to be very interesting," Pieterse said . </P> <P> Alison DiLaurentis is introduced as a very popular girl from the suburb of Rosewood, Pennsylvania, and the leader of a clique composed by her, Spencer Hastings, Aria Montgomery, Emily Fields and Hanna Marin . The first minutes of the pilot episode show the five friends reunited in the barn - turned - loft of the Hastings family during a night of summer vacation . In the midst of night, Hanna, Emily and Aria wake up to see Spencer arriving back to the barn after leaving to somewhere . Spencer naively announces that she has heard a scream of Alison, seemingly shaken due to something . Shortly after, Alison is pronounced as a missing person . One year later, the friends reunite and start to receive threatening text messages from someone unknown using the alias of "A ." During the entire first season, Alison appears in flashbacks . In these flashbacks, Alison is shown as a ruthless, despicable, two - sided teenager who uses the others' secret against themselves . </P> <P> After Ian Thomas is pushed down by "A" from the top of a bell tower and his body goes missing, the girls are forced to keep their friendship a secret and to maintain therapy sessions with Dr. Anne Sullivan to discuss about Alison's absence in their lives, a fact that their parents believe to be the reason why the girls are behaving strangely . Days after, through Spencer's sister Melissa Hastings, the girls find Thomas' dead body, accompanied by a suicide letter in which he admits that he killed Alison . Following, the girls continue to be threatened by "A ." Garrett Reynolds is arrested for Alison's murder after Jenna Marshall hands the Rosewood Police Department a missing page of Alison's autopsy . The second season ends when Mona Vanderwaal--a Rosewood High student and one of Alison's victims of bullying when she was alive--is revealed as "A ." </P> <P> The third season is set five months after the previous one . One night, Spencer, Hanna, Emily and Aria get together in Spencer's living room to drink and enjoy a life without a stalker . Emily, however, is still mourning over the death of her ex-girlfriend Maya St. Germain, who died in the same night of Mona's reveal . After drinking more than expected, the girls wake up in the midst of night to discover that Emily has disappeared . They later find her in the cemetery, standing on Alison's graveyard, with Alison's body gone . With Emily drugged and they involved in a crime, the girls then promise to keep the secret . They dispose of all evidence connecting them to the happening, including all of Emily's clothes . A new person using the "A" identity soon restarts the stalking game . The new stalker threatens them with what happened previously . Mostly of the season's episodes revolve around the question of where Alison's remains are . The girls later on befriend new resident CeCe Drake, a woman physically and psychologically similar to Alison . Drake reveals to them that she and Alison were best friends when Alison was alive, years prior . Throughout the following episodes Drake gets intimate with the girls, who are suspicious of her Alison - like behavior . Later on, Alison is shown in more flashbacks of the night she disappeared, with some revealing that Alison met with CeCe, Aria's father, Jenna Marshall and Garrett Reynolds that night . In sequence, the girls discover, in Alison's biology notebook, a conversation between her and a friend in which Alison talks about a "beach hottie ." The girls investigate while a figure using a red coat starts to appear periodically to them, and they begin to believe that it is Alison . In the meantime, Mona and Spencer's boyfriend Toby Cavanaugh are revealed as helpers (and double agents) of the new "A ." With the help of the two, the girls plot a trap to catch "A," but the stalker uses their own setting against them, and, during a fire, the girls are saved by Red Coat . Hanna wakes up and sees Red Coat staring at her, and the person is revealed to be Alison . Afterwards, Mona and Spencer claim that they also saw Alison . On the way back to Rosewood, Mona reveals that Red Coat would sometimes wear a mask that looked like Alison's face, meaning that Red Coat could have just looked like Alison . </P>

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