Eldest of Lord Borros Baratheon's four daughters by Elenda Caron — the sisters whom the singers would call the Four Storms — born and raised at Storm's End in the easy years before the Dance. By her father's reckoning she was less pretty than her sister Floris and less clever than Maris, but she was the eldest, and when in late 130 AC Queen Dowager Alicent Hightower sent terms south to Storm's End it was Cassandra whom Lord Borros agreed to send north for King Aegon II, widowed of Helaena, to wed and crown his queen. The match was set, the ladies of the Red Keep were already at her gowns, when the king was poisoned by his own small council; word was brought to Storm's End that the king was dead and the betrothal undone, and Cassandra wept bitterly that she would not be a queen.
In 131 AC her mother sent her north again with Ellyn and Floris to stand for House Baratheon at the coronation of Aegon III and his wedding to Princess Jaehaera, after which Cassandra remained at the Red Keep as a lady companion to the young queen. There in 135 AC she became one of the inner conspirators behind the attempted poisoning of King Aegon III and Queen Daenaera Velaryon, a plot dressed up to throw blame upon Lady Larra Rogare; when the conspiracy was uncovered Cassandra was hauled before the regents and confessed at trial in 136 AC that she had shared her bed with Ser Mervyn Flowers and, at his bidding, with the sellsword Tessario the Tiger, the spider at the heart of the poisoning. For the sake of her name and her birth she was spared the headsman: Lord Lorent Grandison and three other stormlords arranged her marriage to Ser Walter Brownhill, a minor knight thirty years her elder who had buried three wives and sired sixteen children upon them, and she was packed off to keep house for him and his brood, her mother Elenda having concluded that nothing else in the world would so swiftly put an end to her scheming.