Ok, so, Marlene has said that to find Big A you had to start watching from season 3 onwards, and who was introduced in S3? Cece. Cece is A, she has always hated the girls/Alison for getting her kicked out of university, she knew the whole time that Alison was alive after she went missing and she (Alison) told Cece that Mona had been A, when she'd worked it out.
Cece went to see Mona at Radley (which we've kind of seen in the show, with Wren admitting he let cece into see her saying she'd be a good influence blah blah blah), and when she visited, Mona (who was doped up on her drugs) thought she was Alison (which she said in the show), which is why at the end of S2 when you see Red Coat visit Mona, she says "I did everything you told me too" she was talking about the advice Alison had given Mona about becoming cool and how to lose her nerdyness. Whilst Mona was off her face she accidentally fed Cece all the information she needed about the girls, which is how in S3 she'd known that Hanna used to shoplift, without any of the main girls saying anything. CeCe was on Ezra's payroll because she would give him enough information for him to get close, but she was always one step ahead of him. ALWAYS. Also, notice how CeCe has never been harmed.
We've seen Cece be revealed as a Red Coat and a Black Hoody (Marlene has said viewers are going to realise how obvious it was when we actually find out who A is) Both Cece and Alison were redcoat, Alison asked Cece to go around in a red coat to try and distract A, because she too went out in a red coat, obviously Cece being A meant that she wasn't being thrown off track by anything and it just meant she could watch Alison more closely without Alison thinking she was in danger, Cece was the redcoat that visited Mona in Radley, and i'm guessing she was also the one who dug up "Alison's" grave in the S3 premiere
Both Alison and Cece were at the lodge in the S3 finale when it burnt down, Alison pulled Hanna out and i'm guessing Cece the other girls (because she hadn't finished torturing them) Cece lost her red coat in the fire and was wearing an Ali mask when she pulled the girls out so that they wouldn't see who she really was, which also explains why in the S4 premier the Alison mask worn by black veil/widow at Wildens funeral was burnt.
NoT to mention the penultimate episode of S4 we see Mrs D handing clothes in a bag to somebody (the clothes which Hanna's mom had held up the episode previously) and then in the following episode, we Cece wearing that exact shirt. So clearly Mrs D is protecting Cece because must have hit Alison and blackmailed her into burying Alison (maybe that's something to do with Bethany?), during the finale we find out that Mrs D has been killed and buried, and her final email read "I can't protect you anymore" who was she protecting?
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