William was born in 985 to two muggle peasants, who, along with other serfs, lived in a small village on the land of a Lord. Both of his parents lived a hard, working life, and expected William to do the same when he grew up. As soon as William could walk, his parents would take him out on the field as field help, usually sowing. These duties from a young age gave William a strong work ethic, with a slight hatred of abusive nobles. Despite his peasant status, like all the other children that occupied the land, the Lord's wife taught him to read and write.
William's first magical act came at the age of eight. He has been awake since 3 AM and was exhausted from working on the field all day. He had sat down to rest under a tree when the Lord came by and saw him, and began yelling at him. William, being incredibly annoyed, attempted to ignore him, however, his shovel magically smacked the Lord upside the head and knocked him out. He was later punished for the magical outburst, however, in the form of more grueling work.
When William turned 11, he and his parents were taken aside by a man who introduced himself as a member of the Wizards' Council. He explained the wizarding community to the three, and offered a position at the newly founded Hogwarts School of Witchcraft and Wizardry. When September came, the same man appeared at the village, taking William to Hogwarts via apparation. William, as he grew up in a peasant village, was amazed at the sight of Hogwarts once he arrived. Like all the other students, he was individually taken aside during the Great Feast, and was talked to by the founders, and was eventually 'picked' by Godric Gryffindor.
William was not a very good student. He was bored of the memorization in classes, preferring classes that were more hands on. He often dueled other students for fun, and was very reckless, often accidentally destroying belongings of other people, much to the founders' and professors' annoyance. Once he graduated, he traveled to the south, throughout many Lords' lands. Eventually he came upon one which was delighted in his magical ability, and made him a knight, in hopes that said magical abilities would enhance him.
William served as a knight for this Lord for many years, until one- fateful morning. He came upon a crowd of peasants who were protesting the treatment by the Lord. In his efforts to disperse the crowd, he was knocked over by one of the serfs with a scythe. A woman attempted to bash him with a hammer, however, it was too heavy for her, and it fell onto his leg, crushing it instead. Then another serf picked up the hammer, bashing him in the head with it, ending his life.
William, not being very satisfied with the way that he was killed (thanks, ye bloody muggles), refused to just pass on and came back as a ghost at Hogwarts. He spent the next 11 centuries wandering the ever-growing grounds of Hogwarts. During the Second Wizarding War, stayed behind in the castle to try and distract the youngest of the students from the chaotic battle going on in various parts of the castle and outside. Ever since then, when there has been chaos, he would try to redirect the youngings away and try to entertain them, even if he fails at the latter, due to his 11 centuries old sense of humor. William also spends most of his time nowadays wandering in an out of classes.