Robert Manderly was the Lord of Highgarden and former Commander of the Golden Company. He played a key role in the victory of the Lannisters, but commited treason to the crown when he slept with the King's wife.
Robert Manderly is the second grandchild to Bartimus Manderly and twin brother to Willas Manderly, who would succeed his grandfather as the Lord of White Harbor. Robert and Willias were close as children, but Robert was always jealous of Willias, knowing he would never inherit his grandfather's chair. He also had a younger brother Wyman Manderly.
Robert, barely a man grown, was still living in White Harbor but his brother became too preoccupied with his new position as a Lord to talk to him or even practice in the yard with him. Robert became close to his bastard brother Brandon Snow, as a result. Snow was a years older than Robert or Willias and had more worldly experience than either. He told Robert about The Golden Company.
Snow was planing on going to join them since there was little else a bastard could do, other than take the black, and he gave the idea to Robert. They took a ship to Sunspear where the Golden Company did their recruiting. They were accepted and sent to Bloodstone, the current base of the company.
Ten years passed and Snow and Manderly became high officers in the Golden Company. Commander Ned Rivers was a brave fool and would go into any combat he could find, regardless of if it was for a contract or not. He died an early death, opening the Lord Commander position. It went to Manderly with little debate. Manderly has been the Lord commander of the Golden company ever since.
After Lord Gylen Hightower purchased the Golden Company, Robert Manderly attended the Council at Bloodstone where Hightower swore himself to Aerion Blackfyre's cause. Some of the Golden Company defended Oldtown during the War of the Ascent of the Lion, and Robert led others to King's Landing where he commanded them under a newly crowned Damon Lannister. While in the capital, he began an affair with Damon's wife, Aeslyn Targaryen.
Under Damon, Robert fought in the battle of Stonehelm where he captured Martyn Dayne in order to force a surrender from his brother Ulrich, the Sword of the Morning, ending the rebellion in the Stormlands.
When Gylen Hightower sacked and burned Highgarden, King Damon insisted on allowing the last living ladies of House Tyrell to retain residence in the castle in hopes of keeping peace in the Reach. Gylen urged the king to place Robert as its new lord in order to keep the last Tyrells in line. Damon ageed, arranging the marriage of Robert Manderly and Meredyth Tyrell.
King Damon eventually discovered that his wife had been having an affair with Robert. Aeslyn fled Westeros and the king married her sister, Danae Targaryen. As a belated wedding gift for her husband, Danae coordinated the poisoning of Robert Manderly with Meredyth Tyrell and her maester brother, Olyvar.
Robert was poisoned, but appeared to have simply fallen ill.
Robert died near the beginning of the third era.
"Robert Manderly, son of Bartimus Manderly, the late Lord of White Harbor. Formerly the Lord Commander of the Golden Company, currently Lord of Highgarden under the paramouncy of House Hightower. In battle he uses a Valaryian steel arakh, sometimes a dagger. He fought with the Golden Company for over a decade. Likely killed more men than anyone could count. He fought at my side at Stonehelm and at the Kingswood, and he was fucking my wife for most of our marriage." - Damon Lannister pointing out Robert to Danae Targaryen
"Lord Robert fought as hard as I to get you your throne. I understand your caution, obviously, but I would feel very disappointed if we let a friend as good as him die when we could have done something." - Lord Gylen Hightower to Damon Lannister
“Robert Manderly may have been a competent warrior, but he was an arrogant fool of a man." - Danae Targaryen
Bartimus Manderly, grandfather (deceased)
Willas Manderly, twin brother (deceased)
Wyman Manderly, younger brother (deceased)
Brandon Snow, brother
Androw Manderly, cousin (deceased)
Meredyth Tyrell, wife