Who is Tyrion Lannister from A Dance with Dragons: A Song of Ice and Fire: Book Five and what is their importance?

Tyrion Lannister is Jaime's brother. He is a dwarf with no nose, which makes him quite conspicuous. Throughout most of the novel, Tyrion appears to have lost all hope and is unable to see a purpose for living. All of these feeling disappear when he has the opportunity to join the Second Sons.