I imagine she used to be a very happy go lucky Legally Blonde kind of gal but afterwards it forced her to mature and a more withdrawn, reserved person. She lost a lot of good friends and now has trouble finding new ones, and she would've went through a few years of therapy until she was functional enough to be in public again.

The killer was still at large and one of the reasons she went into law enforcement was to track them down, but by the time she had entered her career the case had become cold, she hit a brick wall in the investigation and was simply had to settle with the fact that sometimes, terrible things happen to you and never get resolved. So she tries to help other people instead. 

As for quirks. She sleeps with a firearm under her pillow, has a heavy distaste for blood and violence. Has to take medication and see a therapist on sundays. It has definitely effected her life in a significant way, though I know she's also apart of a professional team and would have to have some level of stoicism to simply keep working efficiently.