Willem Kmetsch/History

Birth
500 years ago, Willem was born to a race called Emnetwhits. Willem hailed from an orphanage in the small City of Gomag which was at the edge of Imperial Territory and had a population count of about 3000. For some reason, he was left to be raised by the orphanage. As Willem grew up, he eventually became the main caretaker for the rest of the kids, including Almaria Duffner.

Meeting Lillia Asplay and Souwong Kandel
During his training, Willem met and trained alongside Lillia Asplay and Souwong Kandel under the tutorship of his mysterious master - former 18th regal brave and owner of the orphanage, Nils D. Foreigner. Willem continued training in various arts and skills, but he could never learn higher level abilities due to his lack of natural talent. Willem describes it as "a wall he can never cross."

However, because Lillia didn't want him to become a Regal Brave, out of concern for his safety; Willem was only allowed to become a Quasi Brave.

Becoming a Quasi Brave
Willem dreamed of becoming a brave at a young age. Under the tutor of his mysterious master - former 18th regal brave and owner of the orphanage Nils D. Foreigner - he becomes a Quasi Brave.

As a Quasi Brave, one of a group of elite warriors that served the Church of Holy Light along with the Regal Brave, Willem became known as the strongest of the Quasi Braves and was given the nickname 'Black Agate Swordsmaster' as a result. Though this was later revealed to be a joke made by his comrades.

Final Battle With The Poteau
During the final battle against the Poteau, Willem fought against Ebon Candle who was an immortal earth god and one of the three protectors of the Visitor, Elq Hrqstn along with Souwong.

Using a mix of multiple Carillons and curses, Willem was eventually able to beat him, but at a great cost. Due to the mix of curses that he used, a backlash effect occurred and Willem was petrified as a result. After that, Willem slept for 500 years, unaware that one year later, the Emnetwihts would fail in their attempt to save the world and be eradicated.

Being De-petrified
500 years later, Willem's petrified body was discovered by surface excavation team led by Nygglatho Astartus and Grick Graycrack under a heavy sheet of ice. The team dug him out and took Willem back to Règles Aile to be defrosted and de-petrified. They were successful in the attempt, however, upon waking up in a strange new world, Willem started panicking and freaking out because he didn't understand what was going on. The team and doctors also weren't able to communicate with him either.

Unable to properly communicate with him, the team and doctors were at a loss of what to do, that was until they saw the Babel language translation talisman hanging from his neck. The team then used that talisman to calmly tell Willem what had happened over the last 500 years and about the destruction of the Emnetwihts. Distraught and traumatized, Willem was in despair that everything that he had fought for had been in vain and he felt helpless that he couldn't properly say goodbye to Lillia and Almaria.

The doctors then told him that he had suffered significant damage to his body due to the severe backlash of the 7 forbidden spells that he used when fighting the Poteau, Ebon Candle. The combined effect of the backlash destroyed his body and petrified him. It is said by multiple doctors and Nygglatho herself that it's a miracle he's even alive. It is later revealed in the LN that it's extremely difficult for Willem to even walk due to his frail bones and muscles.

Working for Grick Graycrack
After some time in hospital, Willem was released by the doctors. However, being unable to pay to his medical bills, Willem had to rely on his new friend Grick Graycrack to pay his medical bills for him. Understanding Willem's circumstances, Grick agreed on the basis that Willem paid him back. Grick then took Willem under his wing, taking him on jobs and explained the situation in Règles Aile (its history, economy, food, the culture, the Guardian Wings Military, the political situation of each island and the prejudicial bias against Disfeatures etc.) to him.

This continued for over a year with Willem slowly paying back back his debt, however being still depressed, Willem gave up any thought of being happy and just continued to wait for his death.

Meeting Ebon Candle Again
Technically speaking he won the fight around 500 years ago due to Willem completely obliterating the Poteau's body, causing the god to die on the spot. It's worth mentioning however that Poteaus, being the immortal gods that they are, do not die permanently. Ebon Candle is seen to return in episode 6 of the anime as a giant black skull (He still hasn't completely recovered from the fight before) on a wheelchair as a guardian of Règles Aile. Grick's salvaging group found him 525 years after humanity went extinct (526 years after he was petrified), and brought him up to the land in the sky.

When the anime started, he had already been living on the floating islands for a year.

10 Years Later
It's implied that Willem comes back 10 years after his death (which is a coma state), revived with Elq's and Nephren's help to uncurse him from Seniorious (which he had used to curse himself after he became a Beast). As Elq's said "Ebo said it wouldn't be that hard to revive him."

He then reunites with Nygglatho and a blue haired girl named Ryehl who is implied to be Chtholly's reincarnation, in a similar manner as when he met Chtholly.