I know it sounds impossible but let’s look at this potential story:
We know that Cece went to visit Mona at Radley. Wren gave Cece authorization, according to him because Melissa thought that she’d be a good role model for Mona due to their mutual hatred of Ali. We also know that Cece and Ali both wore the red coat so that Ali could visit town and Cece could distract anyone from Ali by being her doppelganger.
The first time red coat visits Mona is right after Mona’s reveal, when she’s brought to Radley. Mona sees someone in a red coat and says that she did exactly what they told her to do. We now know that Mona thought she was speaking to Alison, because she then hallucinated seeing Alison in a red coat in Radley while Hanna was visiting her.
What did Ali tell Mona to do last time they spoke? She taught Mona how to be popular. This was the night that Ali disappeared at the Lost Woods Motel. Mona did what Ali suggested and she became a queen bee. She became the new Alison DiLaurentis.
Once Mona was brought to Radley, she could no longer be that beautiful it girl. She was tarnished. So she had to become a new version of Ali. When she saw Cece in the red coat, she thought it was Ali and decided that she would become like her. And so evil red coat was born.
Charles got into Radley, probably because he knew his way in and out as a former patient. He taught Mona how to get in and out and they started playing the game together. She told him that Ali came to visit her, so together they dug up Ali’s grave to see whether or not her body was inside.
Emily remembers seeing someone in a black hoodie, who was doing the digging, and someone else in a red coat. That’s all she remembers. Mona claims that she was there too and that she saw red coat. That is a lie. Mona was the one in the red coat. Charles was the one digging the grave. Only two bad guys were there that night.
We know it’s pretty likely that red coat is a brunette. Why do we think this? First of all, we saw the blonde wig in A’s lair. If red coat was naturally blonde, they wouldn’t need a wig, would they? Also, in Wren’s drawing of a woman in a red coat in front of a barn, red coat was a brunette.
In the finale, Mona wears a red cape to prom, dressed as little red riding hood. Why would the writers choose to do that? This could be a huge hint.