Summer's father was a former hedge knight from the Westerlands named Ser Prentiss the Valiant, her mother a beautiful Dornish whore. She was born on a ship as her mother made her way across the Narrow Sea to seek her fortunes in Essos, living in Braavos and Lys.
Much of her time in Braavos as a child was spent learning to waterdance from the young bravos she befriended, and eventually from a master. She explored, eavesdropped, discovered secrets, frequented places she did not belong, and picked pockets. In time her father discovered he had a daughter, and when Summer was fourteen her mother sent her to Westeros to live with him. Ser Prentiss the Valiant had by then earned a small keep in the peaceful lands of the Reach for defending a lord's heir from highwaymen. There he gave Summer the name Steelsong when he saw her spar with and defeat a fellow knight.
Summer won a slender Valyrian steel sword from her dancing master Fallon of Lys. He had earned it by slaying Maeron Stormborn, a Blackfyre through the female line. Fallon called it Dracarys, and Summer called it Mercy, but unbeknownst to anyone, this was actually one of the ancestral blades of House Targaryen, Dark Sister.
Summer swore herself to Lord Martyn Dayne of Starfall, and then formed part of Danae Targaryen's personal guard on her way to the Wall and to Essos. Summer saved the lives of both Danae and Grand Maester Orin Baratheon in Valyria, and having provided true and loyal service became Danae's sworn sword and trusted friend.
Summer returned to Westeros with Danae and remained in her service after she was married to Damon Lannister and declared Queen of the Seven Kingdoms.
Summer accompanied the queen to Storm's End to hold it for Lord Orys Connington. While on an excursion where Queen Danae experimented with dragon-riding, the party was attacked by a group of outlaws. Summer saved the life of Ser Tywin Marbrand of the Kingsguard, incurring a serious wound, and was badly burnt by the dragon Persion. She had begun to show signs of improvement but resumed training too early, frustrated by the thought of no longer being able to fight. She aggravated her injuries in a fight with Ser Anders Sand of Storm's End. Her wounds later festered and she chose to stab herself with her sword and throw herself from her window.
Summer left her few possessions, including a dragon tooth she recovered from Valyria and the Valyrian steel blade Mercy, to Queen Danae.
"In our time together you have done many things. You have saved my life, given me sound advice, stayed by my side from exile to Queen, and become my dearest friend. But not once have you failed me." - Danae Targaryen.
"The nights after Summer’s passing soon became the most trying. When she said goodnight to those around her and finally laid her weary body down to rest, Summer’s face filled her mind in the darkness of her bedchambers. Theirs was a strange friendship bonded by the bustle of Braavos, the smoke and ash of Valyria, and the dragonfire unleashed in Volantis. It reached out beyond Essos to her victory over Edric Baratheon at Dragonstone and her wedding day when she watched Summer toast to the new Queen of Westeros. It was a friendship that overpowered the relationship she held with her sister of eighteen years. It was the loss of her constant companion and guardian and one of the only hearts she trusted." - Danae Targaryen