Baela Targaryen
Daemon and Laena's elder twin, wild as her father, rider of the young she-dragon Moondancer — betrothed to Jacaerys to bind the blacks' two houses into one.
The fuller tale
Daemon and Laena's elder twin is, by consensus of both tellings, the wilder and more overtly ambitious of the two sisters — born in Pentos during her parents' years of voluntary exile, and raised from early childhood around dragons, sailors and the general unpredictability of Daemon Targaryen's household.
Rider of the young she-dragon Moondancer, Baela claims her mount with the same impatient confidence her father is famous for, and grows into the war years as one of the black cause's few young dragonriders old enough to be genuinely useful rather than merely symbolic.
Her betrothal to Jacaerys Velaryon, her cousin and stepbrother, is arranged specifically to bind the blacks' two great houses — Targaryen and Velaryon — into a single unbreakable interest, a marriage of policy that both families treat as essential to holding the coalition together through the war's worst pressures.
The page and the screen
Baela's characterization is broadly consistent between the show and Fire & Blood, both presenting her as spirited and capable, though the show's aired seasons have given her considerably more direct screen presence and dialogue than the book's more distant, chronicle-style treatment of the era's younger dragonriders.
In the show, season by season
Season Two. In The Burning Mill — Moondancer's rider shadows the green host from the clouds. In The Red Sowing — Flies patrol over a fleet her grandfather built and her family finally uses.
In the books
Fire & Blood — The Dance of the Dragons. In The Boy King's Peace — Flew Moondancer against a king and lived to see the peace.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The chronicle records no death for this soul — they are living when the tale leaves them, or their ending is a page the maesters have yet to turn.