Powers: She can drain the life and memories of humans. In the case of mutants; she can "borrow" their powers for a limited time.
Past: Had a normal life up until her powers surfaced.
Introduction into movie: She is shown in her room explaining a future trip to her boyfriend. This leads to a "kiss of death" that leaves her boyfriend in a coma for three weeks.
Importance to the movie: She is one of the two main characters. Logan being the second.
See the pictures! Here