To protect
In "A is for Answers", the identity of the person was revealed to be Jessica DiLaurentis, Alison's mother. In "Game Over, Charles", it was revealed that she was protecting her other daughter, Charlotte, who had hit Alison with a rock under the impression that she was Bethany Young.
Jessica didn't know that CeCe Drake and Charlotte were the same people until Alison and her family left for Cape May. Before Alison arrived, Charlotte admitted to Jessica that she wasn't attending classes and instead, was running around Rosewood with Alison and Jason.
Charlotte tried to wake her mother before discovering that she was dead, and sobbed over Jessica and begged her to come back. Later, for unknown reasons, Charlotte decided to bury Jessica in the backyard of the Hastings' house, and filmed it so she could later send it to Alison as a threat to behave.
Background. Mary Drake was born the twin sister of Jessica DiLaurentis.
To protect Charlotte DiLaurentis after she supposedly killed Alison. To get rid of her twin sister Mary Drake.
Spencer was Mary's daughter, who Jessica hated (according to Mary). When she thought Mary's daughter killed her own daughter she must have been livid.
Melissa was unaware of Spencer's adoption and thought they were biological sisters. They are actually half-sisters, as they share the same father but different mothers. They are also adopted sisters since Veronica adopted Spencer. They share a half-brother, Jason DiLaurentis.
Jessica DiLaurentis
(Hey, she did put dirt in Spencer's bed.) But it turns out that Jessica's involvement in the story was part of a much bigger conflict – one that is still unfurling to this day.
It's down to Rosewood's very own serial polygamist Peter Hastings to break the news that the culprit was none other than Mary Drake. Which is made all the worse – not to mention shocking – by the fact that Mary is Jessica's identical twin sister (and Spencer's birth mother).
In "Game Over, Charles" Cece revealed herself as Charlotte DiLaurentis, Alison's adoptive sister and biological cousin, via her aunt Mary Drake. Alison helped Charlotte seek psychological treatment in Welby State Psychiatric Hospital. Five years later, she advocated for her sister's release.
As Jenna is preparing to shoot her again, out of revenge for her and Charlotte, Mary Drake comes in and knocks Jenna out. Mary then cradles Spencer's body and reveals that she is indeed Spencer's mother, making Spencer the biological sister of Cece Drake.
Unbeknownst to Melissa, Bethany was still alive. Bethany's body was found a year later when builders knocked down a gazebo and discovered a female body. The autopsy report falsely identified her as Alison DiLaurentis after an unknown person switched Alison and Bethany's dental records.
Bethany had learned that Mrs Dilaurentis was having an affair with her father. (In her drawings Mrs Dilaurentis is shown as a monster). Charlotte follows Bethany to the Dilaurentis house, believing that Bethany intends to kill Mrs Dilaurentis over the affair.
Jessica DiLaurentis saw who hit Alison with a rock, and subsequently buried her daughter alive, believing she was dead. In "Game Over, Charles", it was revealed that Charlotte hit Alison, mistaking her for Bethany who was going to hurt Jessica.
Officially introduced in the series finale, Alex Drake is both the identical twin of main character Spencer Hastings and the previously anonymous identity known as "A.D." In childhood, Drake was adopted by a wealthy British family that later abandoned her, forcing Alex to bounce around foster homes and orphanages.
Spencer tells Alex that her friends will figure out she isn't Spencer, but Alex informs her she's already fooled them. Revealing she comforted Hanna when she was kidnapped and put in a barn to be tortured, she kissed Toby in front of the house he was building, and had sex with Toby in Spencer's barn.
She buried her alive. This is what Melissa whispered to her father while waiting for Spencer in the police station last season and it's why Mr. Hastings wants her far away from Rosewood.
Her birth mother, Mary Drake, was a patient at Radley while she was pregnant. Jessica, Mary's sister, told Veronica Hastings that Mary was pregnant with Veronica's husband, Peter Hastings's child. Veronica couldn't stand the thought of the child being forgotten, so she agreed to adopt her.
In the books, Peter is the father of Alison and her twin sister Courtney. In the show, he is Jason, Alex, and Spencer's biological father.
Melissa clarifies to Spencer that she stayed at the resort with Ian when she was getting a secret abortion. She further explains the abortion never took place, as she miscarried.
Mary and her twin sister Jessica Drake were born on December 18, 1965, with Mary being born first. Mary stated that they didn't get along because Jessica was jealous of her and tried to turn everyone against her.
The scene where Veronica revealed all of this to Spencer was heartbreaking. In her mind, there was no doubt that Spencer was her child even though they were not biologically related; she loved Spencer and would protect her at all costs. But all Spencer could see were the lies she had been told her entire life.
Jessica buried Alison alive, whom she thought was dead at the time after Alison was hit on the head with a rock by Charlotte. She buried Alison to protect Charlotte, as she didn't want to lose two daughters in one night.