The first person to properly betray Daenerys Targaryen, outside of her brother, is Ser Jorah Mormont. Ironically, he ends up being the most loyal servant to the Mother of Dragons for most of the show (giving his life to protect her and dying alongside multiple others during the Battle of Winterfell).
Once she reached Dragonstone, Rhaenyra was betrayed by Ser Alfred Broome, whose men slew the remainder of her Queensguard. Her half-brother Aegon II then had her fed to his own dragon, Sunfyre, at Dragonstone.