The Viking protagonist of Assassin's Creed Valhalla is Eivor.
Eivor is a Viking from Norway who leads a clan to England during the ninth century. The character can be played with either a male or female presentation, but the protagonist's name remains Eivor. This is why answers that give a gendered version of the name are not separate canonical characters.
Eivor's settlement in England becomes the base for alliances, exploration, raids, and political missions. The story also brings Eivor into contact with the Hidden Ones and the Order of the Ancients, the historical groups that precede the series' later Assassin and Templar traditions.
Basim and Sigurd are major characters, but neither is the game's protagonist. Basim is a Hidden One whose role becomes increasingly important, while Sigurd is Eivor's foster brother and clan leader. Valhalla's ninth-century setting also explains its mixture of Norse mythology, Anglo-Saxon kingdoms, and fictionalized historical figures.