William is a pukwudgie who was rescued by Isolt from a hidebehind and served/befriended her as he was paying back a debt. He refused to tell Isolt his real name so Isolt called him William after her father. He taught her about magical creatures in America and was taken aback at the fact that Isolt could understand the Horned Serpent. When Isolt rescued Webster and Chadwick Boot William relented and said it was the last work he would do for her and left her afterwards.
James Steward named the house pukwudgie after hearing the funny stories his wife told about William. William saved the family from Gormlaith Gaunt after hearing his named called by Isolt (even though Isolt was referring to another William). William and other pukwudgies help out at Ilvermony. There is still a pukwudgie named William at Ilvermony that some suspect is the original William.