Willem comes from an orphanage in a small village which had 3000 people. It is shown that he likes kids, although some people mistake it for "him being into small girls".
Under the tutor of his mysterious master - former 18th regal brave and owner of the orphanage - he becomes a Quasi Brave. He dreamed to become a Regal Brave, but his master told him, he wasn't fit for it. Yet, Willem never gave up his dream. He continued his training in various arts and skills, but he could never master the skills he learned due to his lack of natural ability.
Willem suffered major damage to his body and venom due to 5 curses incurred when he used forbidden arts fighting one of the protectors of the race known as visitors, Porteau, an Immortal God. He won the fight, although Porteau says he would have to sleep for a hundred years in order to recover, which means he couldn't be killed by willem (save probably for a Seniolis user). Somehow, he was frozen as the fight ended and some explorers found him 525 years after humanity went extinct (526 years after he was frozen), and woke him up. He was frozen a year before humanity went extinct, and before the seventeen beasts came to inhabit the land.
When the anime starts, he's already living on the floating islands for a year.