William Sayre was a Pure-blood Irish wizard directly descended from the famous Animagus Morrigan. He and his wife were murdered by his sister-in-law, Gormlaith Gaunt.
William Sayre’s wife, Rionach, was also Pure-blood, but the couple wass friendly and helpful to their Muggle neighbors. They lived in a cottage called Ilvermorny with their daughter Isolt. William appreciated Isolt’s affinity for nature and nicknamed her Morrigan after their ancestor. William and his wife also raised Isolt to share their attitude towards Muggles. However, Rionach’s estranged sister Gormlaith believed the little girl should be kept from any chance of ever having children with someone other than a Pure-blood wizard, and she took matters into her own hands by killing Mr. and Mrs. Sayre in a fiery attack and taking Isolt away to Hag’s Glen.
Isolt named her Pukwudgie companion William, after her father (Pm:Ilvermorny).
When Gormlaith Gaunt attacked Ilvermorny School of Witchcraft and Wizardry and all seemed lost, Isolt thought of her father and desperately called out his first name. Of course her father was deceased, but William the Pukwudgie appeared to and save the day (Pm:Ilvermorny).
Gormlaith Gaunt assumes William Sayre's name when she crosses the Atlantic disguised as a man (Pm:Ilvermorny).
William Sayers is recorded as travelling to Virginia aboard the Bonaventure (source: Bonaventure passenger list), which Gormlaith Gaunt took, disguised as a man and using the name of Isolt's late father.