Lydia is the main antagonist of Barbie & The Diamond Castle.
Lydia was a muse of the Diamond Castle, but was jealous of the other muses, wanting to be the only one. She vowed to eventually have the Diamond Castle all to herself.
Lydia turned her flute into a weapon and hid in a cave filled with dark magic. When her minion Slyder, detected the presence of Melody, she tracked her down, trying to get the mirror so she can find where the Diamond Castle was hidden.
Lydia attempted to trap the girls in a mansion by hypnotizing two residents that lived there, but only succeeded in catching Alexa. Slyder kidnapped Liana and took her back to the lair.
Lydia used her flute on Alexa and made her almost fall off a cliff into boiling hot liquid, but Melody saved her by confessing where the Diamond Castle was. Liana and Alexa followed Lydia and defeated her, and found the Castle. They saved the muses and used their music to defeat Lydia forever.