Consummate Liar: Put her in a diner with her co-workers and she'll stammer like she's in an exam and hasn't studied. Put her in front of an unsub and she'll play him or her like a harp with laments of how lonely and traumatized (and, in one case, fat) she is. Played with; given her background and the way she behaves, it doesn't seem like all of her weeping about being lonely is lies...
Dark and Troubled Past: Hoo boy, shall we count? Kidnapped as a small child by a serial killer, held hostage for months. Heavily hinted-at physical and sexual abuse. Only escaped due to a fire she'd set and ended up severely burned for a few years. Joined the FBI, where she's surrounded by a daily reminder of exactly how horrible people can be.
What You Are in the Dark: Paul would sit by and allow Jason Travers to kill Zora Petticoff and Billy Ray Pope, because Pope isn't a good man and Pope's manipulations indirectly lead to Karen losing their unborn child in a car accident.
Thoroughly Mistaken Identity: First she mistakes a doll for her son, then a fetus she cut from her first victim, then a live baby she cut from her second victim, then she mistakes Paul for her baby's father.