The second son and middle child of Steffon and Cassana Baratheon, Stannis has always lived in the shadow of his older brother Robert. Even when they were young boys, whatever achievements Stannis had, Robert had them first, and had usually done them better.
Sandwiched as he was between the heir of Storm's End and the lively Renly, Stannis developed into a severe, introverted, and solemn young man. He had no real love of worldly things, and became somewhat infamous for his lack of humor. This was only compounded, when, in 278 AC, Stannis and Robert watched from the parapets of Storm's End as their parents' ship went down in a wild storm, taking them with it.
When Rhaegar kidnapped Lyanna Stark, setting off Robert's Rebellion, Stannis faced a choice between remaining loyal to his king, or fighting for his brother. Ever the dutiful sibling, Stannis sided with Robert.
Stannis spent much of the war holding Storm's End while under siege by Mace Tyrell's Reachmen host. The garrison held out through the most desperate conditions, reduced to eating books before the now-famous resupply run by Davos Seaworth. Eventually, a host under Eddard Stark lifted the siege. Some weeks later, Robert tasked Stannis with investing Dragonstone, the last Targaryen holdout, which he did.
At war's end, Robert ascended to the throne and took his newly-rescued love Lyanna as his wife. He awarded Storm's End to Stannis as a reward for his defense of the castle during the war, and because of his status as the king's next youngest brother and heir, at least until Robert had children of his own.
Not long after becoming Lord Paramount of the Stormlands, Stannis took Milah Caron, daughter of Lord Bryen Caron, as his wife. Milah proved to be the only person who was capable of softening the dour Baratheon even a little. She soon bore him a son, Esmond.
Stannis was made Master of Ships on his brother's Small Council, but the realm did not see total peace during Robert's reign. The Greyjoy Rebellion took many by surprise, and the burning of the Lannister fleet by Victarion and Euron Greyjoy was a damaging loss.
Robert charged Stannis with leading the naval counterattack. The decisive battle was just off Fair Isle, where Stannis trapped the Greyjoy fleet between the island and the mainland by attacking down the straits from the north and south. It was a great victory that broke the sea power of the Ironborn and allowed Robert to land troops on the Iron Islands. Shortly thereafter, Stannis subdued Great Wyk in his brother's name.
After the defeat of the Greyjoys, the realm saw relative peace. Stannis resigned as Master of Ships and returned to Storm's End, where he concentrated on raising a family and ruling the Stormlands. Stannis soon had two daughters, Alia and Haylise, and more recently twin boys, Triston and Rolland. Although Stannis has kept aloof of King's Landing, he has always been a faithful vassal to Robert and has served at times as an interim adviser.