Payback (Athena Force Adventure Series)

Highly trained assassin Dawn O'Shaugnessey was hard to kill--but the superhealing powers she'd been born with had become almost a curse. Dawn had learned the horrifying truth behind her origins from the women of Athena Academy, and she'd vowed to help them destroy the scientist who'd made her the perfect killer. But there was a catch. The very genes that allowed Dawn to take a bullet and heal within minutes were self-destructing, and only her creator could stop the damage. Now Dawn had to choose between life and payback....