Lady Rancora is a famous Starland actress who is mentioned in several books before becoming prominent. She's mentioned by Mira in Scarlet Discovers True Strength and is mentioned again in Astra's Mixed-Up Mission and Clover's Parent Fix. Her name references the word rancor, a synonym for hatred. When she was a Starland Academy student, she was named Cora.
She is the main villain of the series, having posed as Lady Stella to fool Ophelia. Rancora then infiltrated Starling Academy as Lady Cordial. She and Lady Stella share a past history as seen in Good Wish Gone Bad. Rancora appears to thrive on negative wish energy and started the energy crisis to gain power.
In Good Wish Gone Bad, it's revealed that she and Lady Stella were once best friends, but had a falling-out due to a Bad Wish Orb. Cora was negatively affected by an extreme amount of negative energy. Prolonged exposure caused her to thrive on it, and she vowed revenge on Stella and Starland as a whole, planning to destroy the Starlanders' way of life. Stella later learned she and Cora were the subjects of a fated prophecy; one girl would have to save the world while the other would attempt to destroy it. The headmistress at the time, Lady Astrid, noted that she couldn't accept Cora back at the Academy, not only because of what she'd done, but because doing so would undermine the prophecy.
In Stealing Starlight she brings Vivica under her thrall, as Vivica is the subject of a prophecy that a dark starling would decide the fate of Starland. Rancora works from the shadows and gives her her teardrop-shaped pendant to assist her in granting a bad wish.