"Pretty Little Liars: Original Sin" wouldn't be a "PLL" spinoff without the notorious "A," the main antagonist who harassed the Liars in the original show by threatening to reveal their deepest secrets. In the beginning, the Liars believe "A" is their former friend Alison DiLaurentis (Sasha Pieterse), who mysteriously went missing. She's the only one who'd know the secrets "A" is torturing them with, after all. But after Ali's body is found, they realize "A" has to be someone much worse. When signing cryptic messages of torment, the villain's signature is always "A" — short for Ali — and sometimes depicted in bright red.
The true identity of "A" isn't always consistent throughout "PLL," but the moniker sticks. Now, in "PLL: Original Sin," texts, notes, and even the first kill of Imogen's mother, Davie, are all signed with an "A," giving the new Liars their first clue as to who their tormentor could be.
In the original "Pretty Little Liars," "A" doesn't become a physical presence until later in the series, but the first three episodes of "Original Sin" establishes "A" as a tall, brooding, masked villain from the start. Another way the symbol is incorporated into "Original Sin" is through transitional graphics highlighting the passage of time, which feature every letter "A" in red.