Nymeria Sand is the second of the eight bastard daughters of Prince Oberyn Martell, known as the "Sand Snakes". Nymeria's mother was an Eastern noblewoman, who taught her how to use a whip in combat, but who later died in battle. Nymeria became very skilled with using her 8 foot long bullwhip in combat.
Nymeria inherited her father's cunning, and is the most observant and calculating of the Sand Snakes. Instead of charging head-on into a situation relying on brute strength (like her older sister Obara), she will stand back and observe her enemy's weak points, then move in precisely for an accurate strike. Her choice of a bullwhip as her primary weapon exemplifies this mentality.
Nymeria dresses more femininely than her martial sister Obara, though she is no less deadly. Nymeria has long flowing hair, and alluring Dornish-style dresses
She is named after Nymeria, the warrior-queen of the Rhoynar who led her people's migration to Dorne one thousand years ago. The original Nymeria unified Dorne for the first time, and is considered the founder of House Martell in its modern form. "Nymeria" is a popular name in Dorne, much as "Brandon" is in the North, after their founder Brandon the Builder. Through her father Oberyn, Nymeria Sand is actually a direct descendant of the original Nymeria.
Oberyn mentions his eight bastard daughters to Queen Cersei.
When Ellaria Sand goes to the Water Gardens to scold Doran for his inaction to Oberyn's murder, she tells the Prince that Dorne must go to war against the Lannisters, and points out that the Sand Snakes, who have the love of their people, agree with her, and that they will avenge their father if Doran does not.
Nymeria, Tyene, and Obara meet with Ellaria to discuss rallying Dorne to go to war against House Lannister. When the conversation shifts to Myrcella Baratheon, Obara reveals information she received from a merchant captain that Jaime Lannister has been smuggled into Dorne. Ellaria insists that they must stop Jaime from reaching Myrcella. They are presented with a choice: side with Doran and have peace, or side with Ellaria and have war. Nymeria, Tyene, and Obara agree to side with Ellaria.
During the attempted kidnapping of Myrcella, Nymeria initially attacks Jaime with her whip but quickly switches to Bronn, aided by Tyene, while Obara drives Jaime away from the others. Obara barks at Nym to "take her" and Nym breaks off from the fight and grabs Myrcella by the arm. Myrcella resists, but Nym silences her by brandishing a knife in her face. However, as she tries to drag her away they are cut off and surrounded by Areo Hotah and the rest of Prince Doran's personal guard. Areo forces Nym and her sisters to drop their weapons, and they, along with Jaime, Bronn and Ellaria, are arrested.
The Sand Snakes share a prison cell next to Bronn, which proves irritating to Nym and Obara as Bronn continues singing "The Dornishman's Wife". When Tyene begins flirting with Bronn, Nym looks bored until he begins succumbing to his poisoned wound; she and Obara watch with amusement as Tyene toys with Bronn's life before finally giving him the antidote.
Ellaria stages a coup in Dorne that claims the life of Doran and Hotah, and sends Nym with Obara to ambush Trystane. Though Trystane chooses to fight Nym, Obara impales him in the head from behind, to Nym's chagrin, who calls Obara a "greedy bitch", as she had been looking forward to murdering her own cousin.
Many months later, Nym is present when Ellaria meets with Olenna Tyrell in the Water Gardens. The Queen of Thorns shuts down Nym and her sisters when they attempt to speak, demanding that the "grown women" converse without their interruption. Ellaria claims that Dorne and The Reach must work together for justice and vengeance, as Queen Cersei has declared war on both of their factions. Varys appears, claiming that they will defeat Cersei with "Fire and Blood", making it clear that Ellaria, the Sand Snakes, and House Tyrell will support Daenerys Targaryen's claim to the Seven Kingdoms.
After Daenerys and her fleet arrive at Dragonstone, The Dragon Queen and her hand, Tyrion Lannister, decide to send the Martell and Greyjoy fleets to Sunspear in order to stage a joint attack on King's Landing together along with House Tyrell. Nymeria accompanies Ellaria, Yara and Theon Greyjoy, and her sisters on the journey. Below the deck of their ship, Nymeria and Obara playfully tease Tyene. After they are ambushed by Euron Greyjoy and his fleet, both Nymeria and Obara confront Euron while Tyene protects Ellaria. After the Lord Reaper of the Iron Islands kills Obara, an enraged Nymeria attacks him with her whip, only to be choked to death and then hung from the prow of the ship with her weapon, next to her sister.
Nym is one of Oberyn Martell's bastards daughters known as the Sand Snakes. Half-Dornish and elegant, she uses the whip as her weapon of choice. Like her sisters, she supports Ellaria's quest ta avenge Oberyn's death.