When Isolt Sayre went into the woods by Mount Greylock in 1620, she found a Pukwudgie about to be killed by a Hidebehind. The Pukwudgie appeared to be "young and inexperienced" when Isolt saved him by casting a spell with her wand which scared the Hidebehind away. Isolt took the Pukwudgie to her shelter where she helped him recover. The Pukwudgie felt indebted to Isolt for having saved his life, but he was not happy about since he considered being indebted to a young, inexperienced girl humiliating. However, he would not leave Isolt until he had the chance to return the favour and begrudgingly followed her around.
In due time, however, the Pukwudgie came to value Isolt. Their friendship was very unique and he shared with her knowledge of the woods. Since William would not reveal his real name per the traditions of his race, Isolt called him "William" in honour of her late father.
William took Isolt to meet creatures he was friends with or knew of including Hodags, Snallygasters, and Wampus kittens. When Isolt went to a nearby creek, they met a Horned Serpent. William was terrified of the creature, however he was astonished when he saw the Horned Serpent not only liked Isolt, but found it alarming when Isolt claimed the serpent was speaking to her. Isolt eventually stopped mentioning this when she saw that it troubled William.
One day, while he and Isolt were in the woods looking for blackberries, he heard a gruesome noise nearby. He told Isolt to stay where she was while he went to stop it. Isolt followed him anyway and found the same Hidebehind which hurt William had killed two parents leaving their children, Chadwick and Webster Boot hurt and orphaned. After working together to destroy the hidebehind, Isolt asked William to help carry the children to her shelter so she could help them, but William believed they couldn't be saved and wanted to leave them. He then told her that Pukwudgies don't help humans as it is against their beliefs and he only helps her because she saved him first. Isolt then told William that by helping her, she would consider his debt to her fulfilled. William then agreed to carry Chadwick to the shelter. When they got back to their hideout, Isolt was very upset with William and told him to leave, and William glared at her and Apparated.
Several years later, Isolt shouted William's name when her aunt Gormlaith Gaunt broke into Ilvermorny and was about to kill her husband James Steward. Even though she meant her father, the Pukwudgie believed she meant him and Apparated behind Gormlaith and pierced her in the heart with a poisoned arrow, killing her and saving the family.
Isolt thanked William, not telling him that she really meant her father. William, however, was upset that he was never called once since she told him to leave. Despite being unhappy with her at first, he was confused when James shook his hand and touched when he told him they had named an Ilvermorny house after him in honour of the stories Isolt told him about William.
William then moved with his family to Ilvermorny and they helped them repair the school, while moaning about "having" to do so. He felt that wizards were too naive to look after themselves and agreed to stay as the school caretaker in return for payment in gold.
Pukwudgies still remain at Ilvermorny to this day, working while complaining about it, despite having never been required to arrive at the start of every school year to work. Pukwudgie House still remains one of the four houses. When a student is selected to be in the house, a carving of the Pukwudgie will raise its arrow into the air.
No one knows for sure how long Pukwudgies can live, so no one knows when William died, if at all. However, there still remains an old Pukwudgie called William at Ilvermorny who is protective of the marble statue of Isolt and places mayflowers on her grave every year on the day she died. He is, however, unhappy if anyone mentions it and claims that he couldn't be the same Pukwudgie as he would be over three hundred years old, but some people still believe him to be the same William that was Isolt's companion.