|The title of this article is conjecture based on information revealed in the A Song of Ice and Fire novels or related material and may be subject to change.
Originally she lived in obscurity on the ancestral Targaryen holding, Dragonstone island. Her exact parentage is unknown but she didn't resemble her Targaryen ancestors - instead she was notably dark-skinned (like a Summer Islander).
As the great Targaryen civil war known as the Dance of the Dragons progressed, Rhaenyra Targaryen's faction - which was based on Dragonstone - realized that they had more living dragons than dragon-riders, with six riderless beasts nesting on the island. It is believed that only those of Targaryen blood can successfully bond with and ride a dragon, so Rhaenyra proclaimed rewards for any commoners on Dragonstone who could successfully bond with one of the unclaimed dragons - specifically Targaryen bastards and descendants of Targaryen bastards who had been fathered over the years on commoners of the island by young Targaryen princes.
Nettles was one of only four of the so-called dragonseeds to successfully bond with a dragon, Sheepstealer. She cleverly managed to gain his trust by bringing him gifts of freshly killed sheep until he gradually became accustomed to her presence.
Nettles rode her dragon throughout the rest of the war. After the capture of King's Landing, she accompanied Rhaenyra's husband (and uncle) Daemon Targaryen with his dragon Caraxes, as they hunted through the Riverlands for Prince Aemond Targaryen on the great dragon Vhagar.
Eventually, however, after two of the other dragonseeds betrayed Rhaenyra at Tumbleton, she grew paranoid and began to distrust all of the bastard dragonriders, and sent a letter calling for the young girl's head. Particularly, it was said that Daemon himself had taken Nettles as a lover. Daemon was disgusted when he heard of the order: Nettles managed to depart in safety on her dragon, while Daemon issued a challenge to Aemond to come and face him alone. Daemon and Aemond both later arrived at Harrenhal with their dragons, and in the subsequent Battle Above the Gods Eye both men and both dragons perished.
Nettles and her dragon, however, were never seen again.
In the books
In the prequel novellas, nothing is stated about Nettles's past or family on Dragonstone island prior to becoming a dragon-rider.
Nettles is consistently described as a "small, brown girl". Apparently she was notably dark-skinned: the "Histories & Lore" featurettes similarly depict her as dark-skinned with black/African features. It's possible that she was a Targaryen bastard (or descendant of a bastard) fathered on a woman from the Summer Islands, and happened to take after her non-Targaryen ancestors in appearance (similarly, one of Oberyn Martell's daughters, Sarella Sand, was fathered on a woman from the Summer Isles, but she just happens to take after her mother in appearance). This would make Nettles the only non-white dragon-rider in the history of Westeros (though not the only female dragon-rider: several women in Rhaenyra's faction including Rhaenyra herself were dragon-riders).
At the time she claimed her dragon, Nettles is described as 16 years old and skinny. In dress and personality, she is described as being filthy, foul-mouthed, and fearless. She was also called "Netty". Her cunning and intelligence was best evidenced in how she managed to claim her dragon.
When the call came for "dragonseeds" (Targaryen bastards) on Dragonstone to attempt to bond with the six unclaimed dragons on the island, dozens of potential riders were killed in the attempt. Three of the dragons had known riders before so they were claimed before too long, but the other three were "wild" dragons - they had grown to adulthood never being successfully claimed by any rider at all, and thus were unused to the presence of humans. Eventually many thought it was impossible to bond with a wild dragon at all - until Nettles became the only one to succeed, when she bonded with the dragon known as "Sheepstealer" (a basic descriptive name the commoners of the island referred to him by, given that he had a taste for sheep from their flocks). Nettles realized that simply walking right up to a wild dragon and attempting to mount it was foolhardy (and had gotten many of the other dragonseeds killed), so she cleverly left out a freshly killed sheep every day for an extended period of time, allowing Sheepstealer to become gradually used to her presence.
Her dragon Sheepstealer was actually one of the older and larger ones in the Dance, hatched "when the Old King was young" (Jaehaerys I), meaning he was at least 80 years old - and probably bigger than either Rhaenyra's dragon Syrax or Daemon's dragon Caraxes. He was described as being an ugly mud-brown color - for this reason, not unlike Nettles herself, previous Targaryens had ignored Sheepstealer, preferring dragons with what they felt were more beautiful colors (until he had gone so many years without a rider than none could bond with him).
Nettles participated in the massive Battle of the Gullet, one of the largest naval battles in history. The Season 5 Histories & Lore video only mentions that Jacaerys Velaryon died fighting the enemy fleet, not the larger battle that followed, or that the four dragonseeds were involved. She also later took part in the fall of King's Landing, which was a quick and one-sided affair (again, the Histories & Lore video only mentions that Rhaenyra and Daemon were involved, not the four dragonseeds - though the city fell quickly anyway).
Afterwards, Nettles accompanied Daemon to the Riverlands where they used their two dragons to hunt Aemond Targaryen on Vhagar, who was burning out the entire region in a one-man, one-dragon private war. Despite having the largest and greatest living dragon alive, however, Aemond was at least wise enough not to risk riding Vhagar into a two-against-one fight, so he simply withdrew whenever they neared and moved on to burn out some other part of the Riverlands.
Eventually, after the betrayal of two other dragonseeds, Rhaenyra ordered Nettles's execution - also because it was said that Daemon had taken her as a bedmate (though this was never proven). Given that Daemon was old enough to be Nettles's father, it is not impossible that she actually was his bastard daughter, and this is the reason they seemed to have an unusually close bond. The truth of the matter may only be revealed in the forthcoming sourcebook on House Targaryen called "Blood and Fire", but George R.R. Martin has said he will only finish it after he has finished the main A Song of Ice and Fire series.
In despair at Rhaenyra's orders, Daemon flew to Harrenhal to face Aemond alone - while Nettles is said to have been last seen flying her dragon out east from the Vale over the open ocean. It is generally believed that she committed suicide by riding out so far over the sea that Sheepstealer was too tired to fly back to land, and both dragon and rider drowned. Nettles and Sheepstealer were never seen again.
Rumors abound, however, that this was just a ruse to hide from Rhaenyra, and in truth Nettles later doubled-back to land, and that Daemon miraculously survived his fall into the Gods Eye lake, and the two lived out their days together on the hidden Isle of Faces - this tale is popular among the romantic singers. Other rumors speak of a "Woods witch" living up in the Mountains of the Moon, who possessed and old dragon, where they were worshiped by a splinter faction of the Hill tribes that became known as the Burned Men.