His master was a kind man, or so Kin believed at first. He thought Kin how to mix reagents, how to concoct and brew and distill and Kin thrived at it. He soon became adept at the creation of bottled fire, of light in a flask and even ice spheres. He soon learned the intricacies of explosives, of the correct mixing of powder to cause death and destruction. And that is when the pain began.
Fire and wood do not mix, not in a pleasant way. Though he was not truly wood, the pain still became unbearable, unrelenting. His Master forced him in to more dangerous situations, making him destroy his brethren up close, making him cause mayhem and destruction wherever they went. By the end of their time together Kin was covered in deep scarring, his arms, his face and his legs pitted beyond repair. When the revolution came, Kin rose up with the others, killing his Master in cold blood, treating him to a concoction of alchemy and poison, causing his blood vessels to rupture and his eyes to leak his insides.
Stripping his master of all of his finery, Kin destroyed the garments and bound his wounds, hiding his scars from the world, burying himself in large clothing and disguising himself as best he could, before heading off in to the world.
So that is just the origins part of his backstory. Kin is meant to be really, really old (Though I don't know how long Wyrmwood live). I feel like any disturbing flashes of his past would circle around his Master, the things he has done, the cruelty he has endured.
As for how he lives in Ustulav, carefully. He resided in the wilderness for most of his time and traded Alchemy and work for anything he needed. He keeps away from people for 90% of the time and only interacts with people he trusts.