If you go to the last level, "escape" just after you cross both lava pits if you high jump up the left wall you can climb it. Continue climbing (watch for bullets shooting you) and dash right when you make it to the top. You can't turn into the raven due to the machines so dash right as much as you can until you find the red skull. At the final wall, wall bounce over it and you will find the two cute characters behind bars.
BloodRayne: Betrayal Unlockables & Awards
Unlock The Concept Artwork
Defeat the game on any level to unlock the "Concept Artwork" link on the main menu.