After attacking Canute in a rage when the latter killed Askeladd, the death of which was Thorfinn's goal for most of his life, Thorfinn was enslaved as a punishment. Four years later he gained his freedom and changed his life. He is currently a traveling merchant working with Leif Ericson and attempting to colonize Vinland.
Thorfinn's appearance is pretty consistent throughout the story. He has bushy hair, old clothes, two knives, and a rebellious, angry face. He is also relatively short.
When Thorfinn was a child, he had a happier face, round with bright, curious eyes, and well-kept hair.
He has a somewhat slight build that belies his strength.
When the series begins, Thorfinn is shown as an innocent and friendly boy who is searching for adventure. He shared a very good relationship with his family and enjoyed stories told by Leif about his adventures. He has a good relationship with his father and looked up to him immensely, shown when he decides to follow Thors by sneaking on his boat to follow him. This would become the foundation of Thorfinn's drive to avenge his father.
However, by the time he joins Askeladd's band, he is a cold-blooded individual, hellbent on his goal to avenge his father. He becomes aloof toward others, unmoved by and uninterested in what they do or thinks of him, choosing instead to keep to himself. This is seen when Thorfinn ignores the invitations from his fellow Vikings to join in on the fun and his refusing to eat dinner with the rest of them.
His flaws lie in his temper and impulsive nature. This is seen when he ignores Askeladd's input despite the benefits it may be able to bring him, which leads to him losing fights against Askeladd. Frequently throughout the series, Thorfinn's reckless nature leads to his own suffering and failure. Even if he does possess talent far beyond the norm for his age, his hot-headedness is easily his biggest weakness.
However, after the death of Askeladd, Thorfinn went through a time where he was extremely submissive, depressed and without any force, which lasted until the end of the second arc when recovered his courage, and became a gentleman, kind and peaceful. Only resorting to violence as a last resort, when even running away is not an option.
He lived with his family and was a happy, lively boy. He played with the children in the village especially in mock combat with a wooden sword and shield, even though he was always one of the first to lose. One day Floki, and his Jomsviking crew come to Iceland to get aid from his father, Thors. When his father was going out to a battle, Thorfinn hid in a barrel on his father's ship. A mercenary called Askeladd, who was hired by Floki to take down Thors, came and dueled Thorfinn's father. Thors took down half of Askeladd's men, including his second in command, and then the two faced off. Thors won the battle, but in exchange for Thorfinn's life, he was killed by many arrows from Askeladd's men. After his father died, Thorfinn vowed to seek vengeance for his father, by becoming one of Askeladd's men and dueling him every time he completes a mission in combat, so he can avenge his father's death in a fair battle.
"No he is the real thing, for sure. A fighter that agile is rare to come across...." --Garm
Overall Abilities: Thorfinn grew up on the battlefield, having experienced it following his father's death and, as such, has a wealth of battle experience. He quickly grew into a highly skilled warrior capable of swiftly killing warriors at least twice his age. As Thorfinn's fight style plays focus on striking vital points instead of clashing with blades directly, he developed a nimble style that emphasizes speed. Thorfinn's strength is recognized by fighters such as Thorkell the Tall stating he was capable of becoming the next head of the Jomsvikings.
Dual wield: Thorfinn's fighting style emphasizes speed combined with the dual-wielding of two knives, one of which was formerly owned by Thors. By targeting his opponents' weak points and taking advantage of his speed, he could typically finish off an opponent swiftly while avoiding their attacks.
Knife throwing: Thorfinn is also able to effectively use throwing knives, although they are not his primary weapon.
Enhanced durability: Although he doesn't usually engage in unarmed combat, Thorfinn knows how to take solid hits if given the chance to prepare for them. This ability appears when he withstands 100 blows from Drott to earn the right to speak to Canute. However, when set upon by Ylfa in a rage, he was completely unprepared and was knocked out.
Enhanced Agility: Thorfinn's agility is amongst the fastest in the series, he is capable of evading a platoon of soldiers within seconds on a battlefield, as well as scale a wall of a fortress, including jumping over a moat. In combat, he can use his agility to strike from an angle where he may finish his opponent and to find his balance quickly after taking a hit or dodging.
Enhanced Speed and Reflexes: Thorfinn's speed is regarded as his primary asset in combat. During the invasion of London, he earned Thorkell's praise despite losing to him in combat. His reflexes have enabled him to dodge and react to surprises attacks such as Snake's drawing attack. Although, he was not able to dodge Garm's spear. Despite Garm proving faster than Thorfinn as well as wounding him, Thorfinn was nonetheless able to dodge his movements with his eyes. At Jomsburg, he was able to cover a distance of several meters before his pursuers could notice he had run away from them. A testament to Thorfinn's reflexes is shown when he was acknowledged by Garm as being the only fighter he has met to dodge his spear so quickly.