She was played by Sarah Strange.
Justine was a college friend of Alex Moreau (one of the protagonists), and she and Alex encountered each other on the latter's visit to New Orleans. At that instant, Justine reveals herself as a vampiress, and inserts her fangs into Alex's neck, beginning Alex's slow transformation into a vampire herself. The villainess appeared to Alex two more times during her process, feeding on Alex once again and later appearing in her bedroom and allowing Alex to feed off her.
Later on, the evil Justine watched as Alex moved in on Nick, only to be stopped by Philippe, another vampire. Justine informed Philippe that Alex is hers now, but at that moment, Philippe shoots a wooden stake into Justine, killing her.