The bastard daughter of Prince Aemon Targaryen and Elisia Dayne, Aemon (true to his nature) sired her at the age of 14, and he never so much as saw her or spoke to her. Indeed, he never even witnessed her birth. The shame she brought to House Dayne was no concern of his, and likewise for her mother. She was conceived during a royal visit to Starfall and raised there in the castle, never knowing friendship or respect from anyone at all. Until the royal party passed near to Starfall on another journey that was due to end at Sunspear, while Rhaenys was six years of age.
Rhaena Targaryen, Queen Daenerys's eldest daughter and second oldest child slipped away from the party and rode straight up to the gates of Starfall, identified herself, and demanded entrance. Being a princess the guards could hardly refuse her, and all that is known of what transpired is that she definitely visited Rhaenys. Some say she wanted to turn this unformed bastard girl into a tool against her despised older brother. Some say she was simply seeking a small victory in their endless rivalry, and that turning Rhaenys to her side of things would bring her petty pleasure. But it seems that whatever her motivations, she gave Rhaenys the thing she had never had. A friend, a relative who didn't scorn or despise her. Someone to look up to, and Rhaena Targaryen was certainly that. (The only person Rhaena was known to be genuinely close with was her sister, Visenya)
Even at the age of 18 Rhaena already had quite the reputation, her exceptional fighting skills (which she had proven to the court by besting a knight of the Kingsguard, chosen at random) combined with her controversial vow of celibacy and widely known disdain for men had only amplified the imposing presence she had commanded from a young age. Arguably one could say she was looking to mold Rhaenys in her own image, seeing as she would never have children of her own (to her great satisfaction, it must be noted that her immunity to marriage was hard won from her mother).
Rhaena returned to the royal party later that night, and when asked by her mother where she had been, and what she had been doing, she simply replied : "Something i ought to have done years ago, honestly" and refused to elaborate. (Always the most distant from her mother, though she greatly admired her for reclaiming the Realm, as Daenerys had raised her children not on songs and tales, but stories of the horror and reality of what had happened to their family). A definitive effect of whatever interaction Rhaenys and Rhaena had was seen rather quickly. Rhaenys became increasingly rebellious, difficult to control and, one night, even climbed out her window and scaled the entirety of Starfall. She had changed, that much was certain.
When Rhaenys was of the age of 9, shortly after Rhaena recovered the legendary Valyrian blade Dark Sister, and used it to slay Prince Aemon in the throne room (After he had sired twin trueborn children, cleverly), the newly crowned Queen Rhaena Targaryen, First of her Name ordered a grand tourney be held in the capital, and that Rhaenys would be brought to Kings Landing along with the party from Dorne. Much of what happened is, obviously, wrapped in mystery. As the security and impenetrability of the Red Keep was greatly enhanced, according to Rhaena's preferences. But she certainly attempted to train the girl in the arts of combat. Her own quick, innovative fighting style was well suited for a female combatant, after all... although Rhaenys was never seen to possess outstanding natural ability, she was far too young. But regardless by the age of 11 Rhaena had confined all of Rhaeny's melee training to the shortsword and they were to be seen in the training yard less frequently.
Predictably, as she grew slowly older the same sense of intelligence, malice and focus that defined Rhaena and Visenya was beginning to show itself in Rhaenys. Clearly gone was the little girl, and the 'Little Dragon' (as Rhaena called her once at a feast) Was waking up. Never a True Dragon, obviously. Given her birth and Dayne blood, she would never ride a Dragon of her own, never be known as Blood of the Dragon. But that Blood lived inside of her, nonetheless.
Years had passed. Rhaenys Waters was 16 years of age and easily recognizable as dangerous. Yet not inhuman, a rather wicked sense of humor to be sure, put there by Rhaena and Visenya, but like them never truly sadistic. Also like them, she was never interested in men. Much the image of Rhaena herself at that age, if different in her own way. Apparently Rhaena thought that was good enough, for she had Rhaenys brought before the assembled court, sat herself on Aegons Iron Throne and proclaimed that Rhaenys was a Waters no more. That she would be known as Targaryen, because she was worthy of the name, and then she ordered that the court kneel, not to her, but to Rhaenys. "Any Dragon commands respect, and one stands before you" As Queen Rhaena put it.
Rhaenys was then publicly presented with a cloak of beautiful fiery red with a black fur collar, fastened at the front with a circular brooch bearing the red Targaryen Dragon, and two unique ornately crafted weapons. a Crossbow of an innovative design meant to be used with one hand, and a short, mildly curved serrated blade of gleaming steel. "A Dragon without wings still has its teeth", Rhaena stated. And then, true to her nature, she chose that moment to announce that Visenya Targaryen, now Master, or Mistress, of Whisperers had been informed through her spies that a new war was brewing, talk varied of a surviving blackfyre, or a surviving baratheon bastard, but doubtless armies were rising with the intent of smashing the "weakened" Targaryens. The death of Queen Daenerys being viewed as an opportunity. Rhaena intended to enjoy this mistake of theirs, and she had the perfect opportunity to teach Rhaenys the ways of war.