Princess Daena Targaryen is the eldest child and daughter of King Aegon III Targaryen and his second wife, Daenaera Velaryon. Upon her birth, her father declared she would be his heiress until such a time as he had a male heir by Daenaera. This happened a year later, when her brother, Prince Daeron I Targaryen, was born. Daena was too young to remember this, though, so she never resented her brother for this.
Daena is willful and wild. She eventually became a dragonrider to the dragon Vermithor, the mount of both her great-uncle Baelon Targaryen, and King Jaehaerys I Targaryen.