At the beginning, Cullen isn't terribly impressed by Lucas Cader's girlfriend, Mena Prescott. To him, she seems like kind of a ditz and he looks down on her:
Her name was Mena Prescott, and she reminded me of the redhead from The Breakfast Club. She also made me uncomfortable by always hugging on me or kissing my cheek, always doing something I assume she thought I would find flattering or sexy, but instead just found annoying and offensive. (1.29)
She just doesn't seem good enough for Lucas on the surface—she even seems fake. But when Gabriel goes missing, Mena's true colors come out… in a good way:
For at least one night a week since Gabriel's disappearance, Mena had shown up at our house with a sack full of groceries and cooked an entire meal, dessert and all. She had also begun helping my mother around the salon, refusing payment, and had hassled the editor of the Lily Press to run Gabriel's photo on the front page until he was found. (11.17)
It turns out that Mena is a good-hearted, generous person who will stick with her friends and loved ones through thick and thin. Through his time of trial, Cullen really comes to appreciate Mena and feels ashamed for the way that he's judged her. Since this book dabbles in matters of faith, one way to understand Mena is as representing the idea that there's sometimes more than meets the eye. For all of the times that this book suggests religious shortcoming or limitations to faith, Mena is here to represent faith as awesome.