"A flat character merely exists to embody a particular quality or personify and abstract idea. These are the characters that serve as plot devices or provide a foil to the round characters. The very best writing includes very few flat characters.
I believe that one of the reasons many readers are puzzled by the actions of God portrayed in the Bible is that they really expect God to be a flat character rather than a round one. Many of us consider the existence of God to be a logical necessity, but this does not mean that God is an impersonal "first cause." It can be easy to think of God as the abstract sum of his attributes, but the Scriptures portray Him as having emotions and motivations."