Thialfi is one of the sworn servants of Thor. He starts out as a peasant boy. Loki tricks him into disobeying the god of thunder. As punishment for his disobedience, he and his sister (Röskva) will have to serve the god forever. We left his sister out of the story for the sake of simplicity (she is a minor charactor anyway).
He is a loyal, albeit simple, lad. He is a fast runner, who impresses the giants of Jotunheim when he races against Hugi, the manifestation of Thought that was conjoured by Útgard-Loki (or was it Skrymir?).
After returning from Jotunheim, Thialfi becomes a trusted companion of Thor in Asgard. He stands by the god’s side in many battle even becomes a giant slayer himself.