Who is a

-A is Cece mannetjeseend, drake in the show

In the boeken of the show? (I haven't read the books, but I've read spoilers. Plus, there are 16 of those f***ers, so I'm definitely not planning on reading them)

Show:
Mona (Original A)
charlotte DiLaurentis (Big A)
Alex mannetjeseend, drake (Uber A/A.D.)

Books:
Mona (Original A)
Alison (Second A)
Nick Maxwell (Third A/Alison's helper)

At first A is Mona who is mentally ill. Then A switched to Charlotte/CeCe Drake. Then when charlotte died A.D. became the bigger and better A. A.D. stands for Alex Drake, Spencer’s twin sister who she didn’t know about until the finale in season 7.