Who is Simon from The Field Guide and what is their importance?

Simon Grace is Jared's twin brother. He doesn't share his brother's anger problems because he spends his energy catching pets and learning about animals. Simon loves to read and supports Jared's quest to uncover what is going on with the strange creature in the house. His feelings and thoughts, however, are often discounted by his mother because as Jared's twin, he is expected to support his brother.