She is a member of the Cabal of Light and Dark.
Much like Dusk before her, Eventide, born Melyna, was a street orphan that turned to a life of crime to support herself. After a short while she became involved in the same group as Dusk, the two becoming strong friends.
Despite this, when Dusk left for Westeros, Eventide remained in Braavos, inducting new members into the group and training them to become competent thieves, burglars and cutpurses.
As the years have passed however, Eventide has grown to miss her childhood friend and has recently arrived in Westeros searching for her. Like Dusk before her, Eventide's talents quickly caught the many eyes of the Cabal, resulting in her recruitment.