From the series.
Stannis was born the second son to Steffon Baratheon and Cassana Estermont, the younger brother of Robert Baratheon and the older brother of Renly Baratheon. His parents died when he was thirteen. He lost his faith in the Seven on that day, vowing that gods who were cruel enough to take his father and mother in sight of their children would never have his worship. Even in his youth he was said to be a serious, dour and humorless boy.
After the death of Jon Arryn, Stannis flees to Dragonstone to gather his strength and plan his next move. Lord Eddard Stark wonders why Stannis left, believing it was due to the discovery of whatever secret Jon Arryn was allegedly murdered for, although not realizing that Stannis was also slighted by Robert's selection of Eddard as his new Hand of the King. Ned, via Grand Maester Pycelle, sends a raven with a polite letter requesting Lord Stannis to return to his seat on the small council. During Eddard's investigation of Jon's death, he discovers that Stannis and Jon had spent a great deal of time together. When he is told Stannis and Jon visited a brothel, when Stannis is normally so righteous to the point of prudish, he gets closer to the truth. Robert dies after a boar hunt and is succeeded by his son, Joffrey Baratheon. Eddard tries to rule as regent, but is outmaneuvered by Robert's widow, Cersei Lannister. The boy king Joffrey subsequently has Eddard executed.