Stannis Baratheon was the head of House Baratheon of Dragonstone and the Lord of Dragonstone. He was the elder of King Robert I Baratheon's two younger brothers, the youngest being Lord Renly Baratheon. His parents were Lord Steffon Baratheon and Cassana Estermont. He is the father of Shireen Baratheon.
Stannis served his brother Robert in his rebellion against the rule of King Aerys. He succeeded in holding Storm's End, which was a critical victory for the rebellion. Afterwards he used his skills as a naval commander in capturing Dragonstone and defeating the Ironborn in the Greyjoy Rebellion. For his deeds he is named Lord of Dragonstone by Robert Baratheon.
Stannis would later start to uncover the truth behind the legitimacy of the royal children, and he and Jon Arryn discovered that Joffery Baratheon and his siblings were in fact the children of Cersei Lannister and Jaime Lannister, a truth which Jon paid for with his life. Stannis would flee the capital and bide his time, after Robert Baratheon passes, he declares himself the true King of Westeros, and act which is mirrored by his younger brother, Renly Baratheon.
Stannis does not receive the same support that Renly receives, but none the less meets with Renly and demands that he bend the knee. Renly refuses and the two brothers meet in battle the following day. The battle is a resounding victory for Renly, who captures Stannis and sends him to the Night's Watch, but spares his only child, Shireen Baratheon.
Stannis arrives at the wall without issue and immedietly receives the support of many on the wall. While he does not go on the great ranging beyond the Wall, he still plays a critical part in holding the wall in the Battle of Castle Black. Thanks to timely support from Robb Stark, the Wall is held against the Wildling army led by Mance Rayder.
With the death of Lord Commander Mormont, Stannis is named the 998th Lord Commander of the Watch after a unanimous decision. Stannis is a very respected Lord Commander, and when news reaches him of something unspeakable beyond the wall, he leads much of the Watch in the Last Ranging. Whatever was beyond the wall is repelled, but Stannis never returns to the Watch. Some say he died, some others say he attempted to crown himself as King Beyond the Wall to continue pressing his claim, very few others say it was the work of something sinister, the arrival of the Others.