Thorfinn spent his early childhood in Iceland after his family fled there, raised unaware that his father Thors had abandoned a renowned Jomsviking warrior past. Born circa 996, his upbringing was innocent and curious. He frequently engaged in playful mock combat with village children, though he often lost. He cherished a close family bond, particularly idolizing his father and showing keen interest in the adventure stories of explorer Leif Ericson, especially tales of the fabled land Vinland across the ocean. Physically, he appeared cheerful with bright, round eyes and neatly kept hair, a sharp contrast to his later self.

A pivotal moment came when Thorfinn, driven by wanderlust and defying his father, secretly boarded Thors' ship. He witnessed Thors' murder during a confrontation with the mercenary Askeladd, who took Thorfinn hostage to force Thors' surrender, resulting in Thors being killed by arrows. This trauma shattered Thorfinn's worldview, replacing childhood innocence with a consuming obsession for vengeance against Askeladd. He vowed to kill Askeladd in a fair duel to avenge his father.

Following Thors' death, Thorfinn joined Askeladd's band as a warrior. Initially passive and surviving on scraps during raids, he committed his first kill in an island skirmish, marking his definitive entry into violence. Over the next decade, he grew emotionally detached, coldly executing missions for Askeladd solely to earn duel chances. His personality became defined by recklessness, a volatile temper, and singular focus on revenge, ignoring internal doubts symbolized by hallucinations of his father. He displayed apathy toward the band's atrocities, like civilian massacres, though occasionally sparing victims when disinterested in killing. By age 13, he was an accomplished yet isolated fighter, described as short (153 cm) with a slight build, perpetually angry expressions, and messy hair.