Her body was destroyed by the Slender Man and The Blind Man took her memories. Zack Waters got her memories when he became a Gifted, but she planted her consciousness into him. While she no longer has a body, Zack sees her as a blonde petite woman around the age of 20 wearing a lolita-type dress.
Eventually Cindy managed to convince Zack that they needed to kill Scia Malkator; Zack's former neighbor and crush before his memories were taken by the Blind Man. However their attempt failed when Black Crow killed Zack, killing Cindy as well in the process.
She seems to be able to plant her consciousness into other people and manipulate Zack's body. How she does either of these things has yet to be explained.