There will be various Targaryens key to House of the Dragon's story: King Viserys I (Paddy Considine), the succession of whom is what leads to the war known as the Dance of the Dragons; Daemon (Matt Smith), brother of Viserys and very much a rogue prince; and Rhaenys (Eve Best), not to be confused with Rhaenyra, who was passed over for Queen in favor of Viserys taking the Iron Throne.
All will be central to the story in different ways, but none so much as Rhaenyra Targaryen, who is named Viserys' heir and becomes the main character in the Dance of the Dragons. House of the Dragon will follow Rhaenyra's story from her early years all the way through to her end, via political machinations, dragonriding, wars, marriages, and much more. Here's what to know about Rhaenyra Targaryen.
In the Targaryen family tree, Rhaenyra Targaryen is the only daughter of Viserys Targaryen and Aemma Arryn, the niece of Daemon, and the great-granddaughter of the Old King, Jaehaerys I. In terms of House of the Dragon's other characters, she is also the stepdaughter of Alicent, and the daughter-in-law of Corlys Velaryon (Steve Toussaint) and Rhaenys. Rhaenyra will have six children: Jacaerys, Lucerys, and Joffrey with Laenor Velaryon; Viserys, Visenya, and Aegon with Daemon. As far as Game of Thrones is concerned, Rhaenyra is eight generations removed from Daenerys Targaryen.
In George R.R. Martin's Targaryen history book, Fire & Blood, which House of the Dragon is adapting, Rhaenyra Targaryen is established as the eldest daughter of Viserys, and the only one of his three children with his first wife, Aemma, to survive beyond childbirth. Inevitably, Viserys' reign has to include the matter of succession, hoping to avoid similar circumstances that led to him becoming King (which required a Great Council to decide). Although the House of the Dragon trailer has Viserys declare that he won't choose between his brother and his daughter, he does ultimately make a choice, naming Rhaenyra his heir, for a few reasons.
Part of this is Daemon slighting him after the death of his son in childbirth, and with Rhaenyra as his only child, it was an understandable choice. However, Viserys would later have sons with his second wife, Alicent Hightower (played by Emily Carey and Olivia Cooke). Despite this, Viserys refuses to name his eldest son, Aegon, as his new heir. Viserys is a good man, but tended to stick to decisions he made (and doing so would have risked making him look weak and indecisive). Plus, he had already started grooming Rhaenyra to be Queen. It was a bold choice, and one with major ramifications.
The choice of Viserys naming (and keeping) Rhaenyra his heir is what leads to House of the Dragon's main story. Part of that will be the relationship between Rhaenyra and Alicent, who start off as friends but have a relationship that increasingly turns bitter over divisions for who should rule the Iron Throne, with Alicent believing it should pass to her sons, not Rhaenyra (who, needless to say, did not share that view). Tensions mount between them, with two factions forming, the "greens" and the "blacks", named after outfits worn by their pair, with the former supporting Alicent and the latter Rhaenyra.
Rhaenyra continues to prepare for life on the Iron Throne, becoming Princess of Dragonstone at the age of 16, and marrying Laenor Velaryon to further connect their houses and solidify her position, with the Velaryons wealthy and powerful. When Laenor dies, Rhaenyra marries her uncle, Daemon, who further supports her claim to the Iron Throne. It's after Viserys' death that the civil war explodes, splitting Westeros in two, with some major houses supporting Rhaenyra and others Aegon. They wage a bloody, fiery, devastating war that will leave House Targaryen weakened, neither particularly well thought of, and a lot of people dead.
Rhaenyra Targaryen is one of two characters in House of the Dragon played by two different actors (with the other Alicent), as Milly Alcock and Emma D'Arcy take on the role. This will play out linearly though, rather than flashbacks: Alcock will start as Rhaenyra, and there will be a time jump in House of the Dragon season 1 that sees D'Arcy step into the part. House of the Dragon's timeline is an expansive one - spanning around 30 years - and, unlike some of the characters, that takes Rhaenyra from her early years to well into adulthood, necessitating the change of actors, but also a means to highlight how the character is changed over time.