Sierra Meets Castielle

Just what is Sierra's backstory? The other's had all shared their stories including Castielle, but no one, not even Castielle knows Sierra's story before they met. Some how Lynnette has to get them to open up and talk to her about their past. A past that is painful to them both. A past that Castielle bitterly regrets but can't change.
Can Lynnette save Castielle from the bitterness that is destroying him or is it, like his past, too late to change?
This is a companion story/short story and needs to be read after Risky and Frisky: The Vampire Liberace to understand and have no spoilers.