Rusty is Samantha's one-eyed protector. He was once a "bait dog" in a local dog fighting ring who escaped and showed up on Sam's doorstep. She originally intended to give him up to the local pound, but fell in love with him and decided against it.
As an artist - especially a portrait artist - you have to become very familiar with the human skull. It is the foundation of the face. If you are ever going to accurately capture someone's likeness, you need to understand what lies beneath their skin.