Johanna Lannister

Lady Johanna Lannister, née Westerling, is an unseen character in Game of Thrones. She was the Lady of Casterly Rock during the Dance of the Dragons.

Biography
When Dalton Greyjoy and his Ironborn attacked the Westerlands, Johanna was able to bar the gates of Casterly Rock, but the Ironborn were still able to burn the Lannister fleet and sack Lannisport. After Dalton was killed, Johanna was able to attack the Iron Islands, where Lannister troops put several of the locals to the sword and burned the villages and longships.

Reference

 * "The Old Way"