Pretty Little Liars is a popular book series about a group of four girls who lose their best friend Allison. In the books, the girls receive text messages from a girl named "A." But just who is A? But, in the tv show it's been changed a lot. Now Mona is being harassed by A meaning she can't be A. So obviously the producers changed it.

There are TWO A's in Pretty Little Liars.

• The first A is Mona V, who discovered all the secrets about the four girls when Mya moved into Ali's old house, and Ali's diary was with the things that Mya moved out of Alison's old room.
• Mona takes this diary and gets her revenge from when the four girls were mean to her and called her a loser. Mona gets killed when she was trying to push Spencer off a cliff. Spencer moved away, and Mona fell down the cliff instead, therefore killing A number 1.
• The second A is really Alison DiLaurentis-The Alison that the girls were friends with was really her identical twin sister, Courtney. Courtney was mentally ill, so they sent her to a mental institution. However, Courtney escaped from the mental institute and sent the real Ali in her place.
• Courtney got rid of all of Alison's old friends so they would not figure out that she was not Ali, and got 4 new friends, Spencer, Hanna, Aria, and Emily. The real Ali escaped from the mental institution and killed Courtney, then buried her in the backyard.
• Of course, when they found Courtney's body, they thought it was Alison, because they are identical twins. Alison then became the second A because she found the diary that Courtney was keeping about the 4 girls and all of their secrets.

Ali had a twin sister name Courtney, Courtney was acting like Ali cause she has mental problems so Ali had to go to a hospital acting like Courtney. Ali got mad at Courtney so she killed Courtney, Ali is still alive Courtney is dead.

This is rather confusing...

The DiLaurentis's actually had three children; Jason, Ali, and a twin sister of Ali's, named Courtney. While Ali lived as the popular girl, there was a school activity called "The Time Capsule". There are many pieces of the flag hidden, and Ali claimed to have one, because her brother, Jason, told her where it was.

The PLLs (Spencer, Aria, Emily, Hanna) were nobodies, and Ali had befriended the most popular girls in school, Naomi and Riley. All four of the girls had showed up that night to steal Ali's flag piece. Courtney (pretending to be Ali) had said that they were too late, as the flag piece had already been stolen (apparently by Jason, as Aria later reveals). Ali and Courtney had switched places, and the Dilaurentis parents didn't know.

Later on, Ali is sent to a mental hospital (the very same one that Hanna had been sent to by her father) and Courtney takes her place in school. Not knowing who Naomi and Riley were, she ran to befriend the 4 PLLs. Ali later gets revenge on Courtney for ruining her school life and pushes her into a deep ditch to kill her, during a sleepover in Spencer's sister's barn.

(side note - Mona probably didn't actually have any interaction with the Ali that killed Courtney.)

In other words, Ali was NOT killed in the ditch.

BUT.

The DiLaurentis parents announced that they indeed had another daughter... one that no one (including the PLLs) knew about. Her name was Courtney (who was actually Ali). She tells all of the PLLs and brings them to their beach house as a "celebration sleepover" for reuniting. While they were sleeping, Ali traps them inside the house, to try to kill them. She gives them a note that says that they ruined her life. This is when the PPLs smell smoke and try to escape the house, using a secret passage that they used to use for scaring Jason DiLaurentis, where they see Ian Thomas's dead body, and Spencer's older sister, Melissa (still alive). Barely escaping, they watch the house blow up, with the REAL Ali still inside.

Sara Shepard leaves the reader hanging in the epilogue, when she says that a new student has moved into your school, as an anagram on Alison DiLaurentis, suggesting that she is still alive.

There are many speculations as to who killed Ali teen series "Pretty Little Liars." Add your thoughts below:

• The Pretty Little Liars never knew that Ali actually had a twin named Courtney, who was insane. One day, Courtney managed to switch places with Ali. The Ali that all the girls knew was really Courtney! When the real Ali finally came back, she killed Courtney.
• YES! Courtney is really the dead one, not A (Ali).

• Ali never died. She has a twin sister named Courtney, who is sick in the head, who pretends to be Ali all along, while Ali is away at a mental home. Courtney becomes "friends" with the four girls and learns about all of their secrets and keeps it in a diary. When Courtney (Ali) is supposed to be coming back from the home to visit her family, the real Ali murders her sister in revenge.

• Okay, in the last book in the series, Wanted, you learn that Alison DiLaurentis had a twin sister named Courtney DiLaurentis, who didn't live with the family due to mental issues. The twins are identical. Later on you find out that Courtney always pretended to be Alison, causing much confusion amongst the DiLaurentis family. The night that "Alison" disappeared, it was really Courtney in Spencer Hasting's barn, not Alison like the girls thought. Alison despised Courtney because back awhile ago, Courtney dumped Alison's popular friend Naomi, and traded her for 4 new "losers," who just so happened to be Hanna, Spencer, Aria, and Emily. Many of the girls adventures with whom they thought they shared with Alison, were actually with Courtney who pretended to be Alison. In the end you discover that the actual Ali killed Courtney for throwing away her perfect life. The body that was discovered in Maya's backyard was Courtney DiLaurentis. Ali is still alive, but after Mona Vanderwaal dies, Ali takes on the role of "A." So in conclusion, Alison is the killer, and she killed her twin sister, Courtney out of fury and revenge. She ended up killing herself by trying to kill Hanna, Spencer, Aria, and Emily.

• The person who was killed was Ali's twin sister Courtney. She was killed by Alison because she had stolen Alisons perfect life