Nyssa Raatko is the daughter of Ra's al Ghul and younger sister of Talia. After her father's Arkham City plans were foiled, she lead a splinter group of League of Assassins to prevent him from returning.
Batman: Arkham Origins
Nyssa's name is mentioned Shiva's profile, though she does not make an appearance in the game.
Before Arkham Knight
At some point, Nyssa met Batman and learned of his true identity, Bruce Wayne. After Ra's and Talia apparently died in Arkham City, she joined the faction against her father's return. Under her guidance, Nyssa was able to cut off the Lazarus Pits and ensure her father's loyalists couldn't resurrect him.
During Scarecrow's attack on Gotham City, Nyssa and her splinter faction were stationed at a Lazarus Pit on Bleake Island. When Batman took a sample of the pit, she confronted him and warned him not to resurrect Ra's. Nyssa destroyed the pit with his Explosive Gel and asked him to let Ra's die. In return, she promised not to spill more innocent blood and to leave Gotham, swearing on Talia's death. As he left, Nyssa followed him to the Elliot Memorial Hospital with a number of her own troops. Depending on whether Batman decides to heal Ra's or let him die, the following will occur.
If Batman decides to heal Ra's, Nyssa and her forces confront him in battle. After they are defeated, she battles the vigilante and her father. During the fight, Nyssa is mortally injured by Ra's. After he escapes, Batman offers to resurrect her with Lazarus, which she refuses. As she dies, Nyssa reflects on his choice not to kill and why Talia loved him. After her death, her forces leave Gotham.
If Batman decides to let Ra's die, Nyssa arrives moments after he defeats her father's followers. Effortlessly overpowering him, she is prevented from killing him by Batman, who offers to take him into custody. Agreeing to his demands, Nyssa promises to fulfil her promise. As she leaves, Batman warns her not to break her oath.