Lydia is an evil sorceress and the main antagonist in Barbie & The Diamond Castle. She is voiced by Kathleen Barr.
Lydia was a Muse who had fallen from grace, having turned against the other Muses for selfish reasons, and sealed her degradation from a good spirit of music into an evil witch by transforming her flute into an instrument of powerful dark magic. Along with Slyder, she travels to capture Melody and find the Diamond Castle.
Lydia was originally a Muse of the Diamond Castle, working closely with the other two Muses - Dori and Phaedra - to take care of the castle and create music. However, she became insanely jealous of her friends, wanting to be the one and only Muse.
Dori and Phaedra disagreed with her, believing that music was for everyone. A scorned and furious Lydia stormed out of the Diamond Castle, with the vow that she would return and have the castle all to herself.
She found an ancient cave filled with magic, and transformed her flute into an instrument of a dark magic so powerful that it could do almost anything she willed it to do. With her dramatically-enhanced powers, she planned to take over the Diamond Castle, and from there conquer the world piece by piece.
Dori and Phaedra hid their own magical musical instruments along with the castle, and entrusted the key to Melody, their apprentice, in case anything bad happened to them. They then went to see Lydia and tried to reason with her, but she simply turned them both into stone statues. Melody managed to narrowly escape by magically hiding in a mirror.
When her minion, Slyder, detected Melody's presence, 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 Princess 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, turning her and Slyder into statues.
- Kathleen Barr also voiced villainesses Queen Zarconda from Extreme Dinosaurs, I.M. Slime from Grandma Got Run Over By A Reindeer, Mesmira from Conan the Adventurer, and Laverna from Barbie: Fairytopia.