Jacaerys Velaryon is the eldest son of Princess Rhaenyra and Laenor Velaryon, though his biological father is Ser Harwin Strong. Laenor knows Jacaerys and his brothers are not his and accepts it, because he's gay and he and Rhaenyra were unsuccessful in having children of their own. In "Fire and Blood," Jacaerys and Daeron are around the same age.
Jace is betrothed to his cousin (and stepsister) Baela Targaryen. He rides the dragon Vermax, who he bonded with when the beast was a hatchling. Jace is the first of Rhaenyra's sons to figure out who their true father is, and he mourns his death. Jace is his mother's heir, and would become king after her rule as queen — if they ever get the throne.