Orphaned when vampires slaughtered her parents, Cassidy Sharp was raised as a ward of the Darkwatch and trained to become an agent. To earn respect in a man's world, Cassidy had to ride, shoot, and fight as well as any man, and think faster than all of them. Haunted by visions of her family's massacre, Cassidy harbored deep doubts about her abilities and courage in the face of the supernatural -- she overcompensated by being the most serious and idealistic agent in the Darkwatch organization and the creeping corruption that threatens to destroy it from within.
In the opening sequence of Darkwatch, Cassidy attempts to stop Jericho Cross (player's character) from opening a vault containing the vampire lord Lazarus Malkoth. Failing to do so, Jericho becomes a vampire. As the sole surviving agent of the Darkwatch train transporting Lazarus Malkoth, Cassidy decides to help him retain his humanity. Resourcefully deputizing Jericho Cross into the Darkwatch, she leads him in a desperate bid to warn her superiors about Lazarus' escape.
Soon, however, she is killed by Lazarus and since then she is aiding Jericho as a ghost (in a role similar to this of Cortana in Halo).