Helaena Targaryen is the daughter of King Viserys and Queen Alicent. Interestingly, she's one of the only people in the Targaryen family tree with a unique name that isn't at least a version of someone else's name. It's possible that she was named for Alicent's mother, whose name is unknown. Helaena is also a dragonrider, whose mount is Dreamfyre, though viewers have yet to see her with her dragon on "House of the Dragon." There are also clues in the show that Helaena is a dreamer who can see visions of the future.
Helaena knows for most of her life that she will marry her older brother, Aegon II, though her half-sister Princess Rhaenyra does briefly suggest Helaena marry her nephew Jacaerys, Rhaenyra's eldest son, instead. Before they're married, Aegon is shown in the series as not being interested in spending any time with his sister, and he mocks her. The couple, in the books, eventually have three children.