She's completely selfish, and treats Perrin more like an unruly dog than a partner. First she messes Perrin up by not simply telling him how much she loves him, forcing him to guess whether she really loves him or if she's just a fling. Her actions and words provide little clue for him, and it's no real surprise when he guesses wrong.

But her actions with Loial and the Waygate are what were really abhorrent. Knowing that Perrin was trying to leave without her, she should have had an open, honest, mature conversation with him about her love for him and her hope that they could be lifelong partners. But INSTEAD, she decided to backstab him by tricking one of his closest friends, making jokes about him being a "puppy" and "follower", and trying to humiliate him by forcing HIM to apologize to HER.

Not to mention her damn nonsense with Berelain. Faile's attack on Berelain in the Stone is what led to Berelain's pursuit of Perrin later, and all the harm that that caused to Perrin with his standing among his army. But Faile takes it out on Perrin as if it were all Perrin's fault.