Watching Justice League Dark...so was Zed...
Supposed to be, or become Zatanna?
Constantine keeps referring to her as Z, and Batman said they have a history.
Nope. Zed and Zatanna are completely different characters. Stage magician and superhero Zatanna Zatara has been around since 1964. She is the daughter of the (originally) Golden-Age hero Giovanni "John" Zatara whose first appearance was in Action Comics #1 which also saw the first appearance of Superman.
"Hell hath no fury like that of the uninvolved." - T. Isabella