Karli is the child of a friend of Lief named Arngrim. Karli's mother died due to an attack of vikings allied with Gisli in his feud with Arngrim.
Thorfinn, after finding him in the wreckage of his own home while being guarded by his half-dead mother, and his crew takes him with them on their journey to Miklagard.
He is a young child of no more than 1 year old. He has ginger hair in the color panels and has a stocky-ish build for someone his age.
Other than this he is just a normal baby.