Flat character:
A flat character is a person who does not change in course of the narrative of a fictional work of literature. The fcatures of a flat character are:
a) He or she remains the same throughout the story of the work;
b) He or she is simple in nature;
c) In most cases he or she plays the role of a supporting character,
In Dickens' Great Expectations, Joe is a flat character because he is simple, plays a minor role and remains unchanged. Similarly,in Jane Austen's Pride and Prejudice, Mr. Collins, Mr. Bennet and Mrs. Bennet do not change. They are simple and minor characters. They are flat characters. (see Round Character)