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

Stannis Baratheon, know as the First of His Name, the Rhoynar, King of the Andals, and the First Men, Lord of Dragonstone, Lord of the Seven Kingdoms, and Protector of the Realm, is desribed as a cruel and ruthless man.