Poor Unfortunate Soul
The third book in Serena Valentino's Villains series tells the backstory of Ursula the Sea Witch. Found as a girl clutching driftwood and raised by a fisherman, she is driven from her adoptive home after townspeople try to burn her as a witch. She later learns she is the estranged sister of the merman king Triton, who brings her to court only to push her out again once she has served her purpose. Embittered, Ursula turns her attention to Triton's youngest daughter, Ariel, and her forbidden love for a human prince.
The Sea Witch; raised by a human fisherman before discovering her true nature and her estranged brother, King Triton.
Ruler of Atlantica and Ursula's estranged brother, who casts her out of his court.