Robert Baratheon is the head of House Baratheon and was crowned King of the Seven Kingdoms after winning the War of the Usurper. He is the first son of Lord Steffon Baratheon and Lady Cassana Estermont.
Robert is the eldest of three children, and became lord after his parents died in a shipwreck. In his youth, he was fostered by Jon Arryn at the Eyrie with Eddard Stark, who would become his best friend and most trusted advisor. Robert fell in love with Eddard's sister Lyanna Stark, whose hand in marriage was promised to him. Despite this, Robert is known as a notorious womanizer and has fathered several bastard children. He was once a handsome and renowned warrior, but he has become immensely overweight after winning his throne.
The war for the throne began after Prince Rhaegar Targaryen kidnapped Lyanna and took her to Dorne. This led to Lyanna's father Rickard and her brother Brandon Stark heading to King's Landing to demand justice from King Aerys Targaryen, who executed them both. Lord Jon then called his banners and both Robert and Eddard rallied to his side. Robert believed he had the best claim among the three to take the throne as he had a Targaryen grandmother. He personally led the deciding battle of the war at the Trident, where he killed Rhaegar.
Although he had won the rebellion, Lyanna Stark died shortly after in Dorne, haunting him for the rest of his life. He married Cersei Lannister to unite their two houses and bring stability to the kingdom, though it was a loveless marriage as he was still in love with Lyanna's memory. This earned the hatred of his new wife, who swore never to bear him any heirs. All three of his children were actually produced by the union of Cersei and her brother, Jaime Lannister, unbeknownst to Robert.
Though Robert Baratheon became king by winning the rebellion, he cared little for actually rulling. If it were it for him, he would have handed the crown to Ned Stark or Jon Arryn believing they would have done a better job, but his claim to the throne was the strongest because (ironically enough) he descended from the Targaryens themselves. He turned out to be poor ruler, leaving the realm in severe debt and spending his time drinking and whoring. It was only Jon Arryn as Hand of the King and his brother Stannis's effort that kept Robert's rule from collapsing. Only in times of war, he turned out to be truly effective. When the Greyjoys rose in rebellion against the Iron Throne, he felt relieved from the boredom in times of peace and with Westeros' full might, they effortlessly crushed the Ironborn rebellion. This act had further solidified Robert's rule.
Fifteen years after the rebellion, he leaves for Winterfell to ask Eddard Stark to be his new Hand of the King, after Jon Arryn dies under mysterious circumstances.