What is most obvious is that Eric is Sookie's opposite in many ways but also the writers show their similarities too (I'll get to that). When Eric and Sookie are in a scene together one of the most blatant ways they contrast them is to dress Eric in all black and dress Sookie in white. We all know black means darkness and evil (Eric is not necessarily evil but that is one way black is used) and bad and white is virginal, innocence and good. It hearkens back to the westerns: good guy in the white cowboy hat and bad guy in the black hat. As we know Sookie is our sweet, fierce and loyal heroine. Eric's character is dark and mysterious, but they are similar in their loyalty to their families (Eric's loyalty to Godric and Pam and Sookie's loyalty to Jason and Tara). In Season 2 "Timebomb", Eric is dressed in black and Sookie is wearing a white trench coat. In Season 3 when Sookie goes to see Eric about the werewolves abducting Bill, she is wearing a white blouse and Eric is once more in black. These techniques show us how these two are each other's foils. Notice also in Season 4 when Eric loses his memory, he is no longer dressed to contrast against Sookie, because the message is they are more alike than different in this season. When Eric's memory returns, he's back in black.
Now let's shift gears. Why is it important to create a contrasting character to foil another one? Sometimes the foil is meant to shine a mirror on the protagonist to show us something about that character. In book two of California Girl Chronicles, I introduce a new love interest named Ryan. Those who have read book one were introduced to our sweet, kind, decent and hardworking producer, Kale. Ryan is Kale's foil. Both characters come from Hollywood money. Kale used his family's money to launch his successful production company. He is mature and grounded. Ryan uses his family's money to play and generally loaf around. Ryan is fun-loving, lazy (unapologetic about being lazy) and wild (the opposite of grounded). Now here is a third layer: Brea likes them both for being the opposite. And in her attraction to the two of them, we have a third foil that contrasts her. What does her affinity to each vastly different character say about her?
As a writer, I have shown you elements to the story you need to think about. When you add elements such as character foils, you are deepening the psychology of your story. Yes, California Girl Chronicles is a fun read, but as I just pointed out, there is much more there then you realize. Readers aren't meant to be distracted by your techniques. Your story should be told seamlessly, but the addition of the layers to the story give it more substance. These points give the reader unconscious messages about what the story is really saying.
If you would like an autographed copy of California Girl Chronicles, please purchase it off the 3L website. You can also get a copy from Amazon.