King William is Odette's father who appears in the beginning of the first film. He devised a plan with the ruler of the neighbouring kingdom, Queen Uberta, to marry off their children, Derek and Odette. Shortly after Odette's birth, he defeated the sorcerer Rothbart and banished him.
He dies when Rothbart attacks him and his guards when they left Derek's Kingdom, and kidnaps Odette. After upon hearing this, Derek arrives and sees William laying on the ground, telling him that "it's not what it seems..." before dying. With William gone, his Kingdom is now Odette's and that is what Rothbart wants.