Season 1-3, Mona. Spencer and Toby both become A for season 3, however they had alterior motives. RedCoat is the one in charge, we have seen Ali as Red कोट and CeCe as Red कोट but thers probably 3 and ali is कहा to be the good one.
This is sorta a spoiler. A is whole group of people. Mainly people who hated Alison when she lived in Rosewood. I know for a fact that A is: Mona, Lucas, Shanna, and many other students that go to Rosewood High. Oh and I believe that Wren and Melissa Hastings are also involved in the A group. Hope that helped a little.
Season 1-2, Mona. Season 3-6A, CeCe. Season 6B-7's A will probably turn out to be Addison Derringer या Lucas, and I'll have to check myself into Welby.
