In the second season of HBO’s Game of Thrones, Brienne swore an oath of fealty to Catelyn Stark. The oath followed the death of Renly Baratheon, whom Brienne had served as a loyal member of his household guard. When Brienne was blamed for Renly’s assassination, Catelyn helped her escape before Renly’s followers could seize her.
Brienne then pledged herself to Catelyn and accepted a new duty: helping return Jaime Lannister to King’s Landing in exchange for the safe release of Catelyn’s daughters, Sansa and Arya. Although the arrangement involved Jaime, Brienne’s personal loyalty was to Catelyn and to the promise she had made.
That vow shaped Brienne’s story long after Catelyn’s death. Jaime later gave Brienne the Valyrian steel sword Oathkeeper and charged her with protecting the Stark girls. The name reflected Brienne’s determination to honor her promises, even when doing so became dangerous or seemingly impossible. Her later service to Sansa grew directly from the oath she made to Sansa’s mother.