Troian Bellisario portrays Spencer

Troian Bellisario portrays Spencer Hastings. Spencer is an extreme perfectionist who likes to please her wealthy family, and her friends. She is very competitive, and an overachiever. Spencer is strong willed, smart, and kind towards everyone around her, but isn't afraid to take down someone who is a threat to something she cares about. Tired of being the victim, Spencer was a brief member of the "A" team in the final two episodes of the third season. She is in a relationship with Toby Cavanaugh (Keegan Allen), who is now part of the Rosewood Police.
Ashley Benson portrays Hanna Marin. Hanna used to have an eating disorder. She was overweight, with the nickname 'Hefty Hanna', and after Alison Dilaurentis disappeared, Hanna lost her weight and changed her style, which seemed similar to Ali's, with the help of her new best friend Mona Vanderwaal. Over the course of the series, Hanna cares more about the people around her and tries to protect herself and her friends. She has had few love affairs through the series, and is in a relationship with Caleb Rivers (Tyler Blackburn).
Lucy Hale portrays Aria Montgomery. Aria is the artsy chameleon who is very intelligent, caring, and has a good sense of style. She's the one who keeps her family together. Aria used to be a tomboy. She would wear black, punk-like clothing, and she had pink highlights in her hair. After Ali's disappearance, Aria and her family moved to Iceland for a year before returning to Rosewood. When Aria and her family came back from Iceland, she was no longer that tomboy she used to be, she was a girly-girl. Aria had a relationship with her former English teacher, Ezra Fitz (Ian Harding) but they decided to end their relationship before Aria went to college.
Shay Mitchell portrays Emily Fields. Emily is the sporty one in the group. She is the best swimmer on the swim team, and she's very sweet and loyal. Emily is a shy, kind 'closeted lesbian', who was in love with Alison DiLaurentis. Over the course of the series Emily came out to her parents that she was in a relationship with Maya St. Germain. After Maya's murder, Emily develops a relationship with Paige McCullers in season 3. They're together for a while, until Paige betrays Emily's trust. Alison confesses her feelings for Emily.
Sasha Pieterse portrays Alison DiLaurentis , former "Queen Bee" of her clique and the most popular girl in school before her disappearance. She enjoys using people's deepest secrets against them to keep them in line. Charming and manipulative, Alison managed to manipulate all four of her friends in some way or another and is skilled at finding ways to blackmail others. It was later revealed that Alison is alive, and was on the run from "A".
Janel Parrish portrays Mona Vanderwaal, the original and first "A". She is the person that stalked Alison before her disappearance, and did this as revenge for Alison crowning her "Rosewood's biggest loser". After Alison disappeared she became best friends with Hanna Marin and they transformed themselves into the most popular girls at school, taking Alison's place. In the season two finale she is exposed as "A" and sent to Radley Sanitarium, where she is visited by Charles DiLaurentis, whom takes up a partnership with her for the "A" game. In Season 3, she continues illegal "A" activities and by the end of the season is kicked off the "A" team and becomes a victim of "A" along with the other girls. In Season 4, she helps the girls out with their investigations of the new "A". The Black Widow places a Mona doll in her dollhouse which means that she is now considered to be part of the Liars. In season 5, she creates an army so as not to be intimidated by Alison's return. Later on in the season she fakes her own death to ferret out "A" but is kidnapped by "A" and imprisoned in "A"'s dollhouse.