Francis first meets her when she discovers Greta’s “lost” bag on the subway, and the two quickly begin a mother-daughter-esque relationship as Francis tries to help her (and it doesn’t hurt that Francis just recently lost her mom and has only just moved to New York City).
Before too long, however, she realizes that the bag was planted to try and lure her in, and Greta’s obsessive personality starts to become excessively disturbing.
From the outside Greta appears to be an innocent woman in need of good company, but as we witness as the story unfolds that’s not really the case.