Askeladd

Lydia (mother)

A cunning, mysterious Viking who commands a mercenary band of vikings, of which Thorfinn is a member.

[Spoiler] Askeladd was contracted by Floki to assassinate Thors, Thorfinn's father. Thorfinn became obsessed with avenging his father and joined Askeladd's band with the sole intent of killing him in a fair fight, but is always outclassed by Askeladd's combat skill. [/Spoiler]

[Spoiler] His real name is Lucius Artorius Castus. [/Spoiler]

Askeladd is a tactical genius and a master swordsman. His ability to manipulate those around him turned a horde of pirates and raiders into a close-knit group loyal to him, as well as proving him capable of turning enemies into allies, as seen with Thorfinn, Thorkell, and Prince Canute.

Askeladd is named after the Norwegian fairy-tale character Askeladden, known for his cunning nature and cleverness.

(Source: Vinland Saga Wiki, edited)