Born in an age where the strong took from the weak, Thialfi's parents were murdered by a rampaging horde of Viking warriors who killed in Thor's name. However, when the real Thor paid the Vikings a visit to voice his disapproval of their actions, the Thunder god took pity on the orphaned child and asked for guidance from his father, the almighty Odin. Odin claimed that the human world is best left to the humans and the gods should not interfere, but Thor felt responsible for the child and insisted Odin make an exception in his case. Hesitant, Odin complied on one condition. He told his son that when Thialfi came of age, Thor must admit him into his circle as his most trusted ally. Thor agreed, and Odin used his power to bless Thialfi with wisdom and insight along with the physical attributes of an Asgardian male. Finally, Odin gave Thialfi the ability of superhuman speed so that he may outrun any enemy. Years later, when young Thialfi became a man and Thor became Lord of Asgard, Thialfi became Thor's advisor much to the dislike of those already in Thor's service. Even after Thialfi delivered the axe of the Skurge the Executioner to Thor so that he may defeat Desak, the Destroyer of Gods, there were those that still distrusted him. Even Balder the Brave accused Thialfi of being a spy of the treacherous Loki. It was only until Balder was reassured by Thor himself to trust Thialfi that the debate came to an end.
However, when Thor became obsessed with ending humankind's suffering by bringing Asgard to Earth and allowing the gods to directly interfere with the mortal world, Zarrko the Tomorrow Man, decided to wage war on Asgard. Together with government soldiers, Zarrko hatched a plan that was meant to destroy Thor. After the attack on Asgard succeeded in destroying most of the golden realm, an enraged Thor gave Zarrko enough of a reason to attempt to flee. Zarrko opened a time portal, but his escape was hindered by falling debris, and Thialfi fell into the portal instead only to be transported into the distant future. Once there, Thor threw a huge party to celebrate the return of his long lost friend, but the reunion would be short lived. Once Thialfi traveled through the realm and saw for himself what the interference of the gods had done to the world of men, he became confused as to what side he should be on. Eventually, Thialfi, along with others such as Thor’s own son, Magni, decided it was their responsibility to stop him. As a weakened Thor was about to enter the Odinsleep to renew his almost depleted powers, Thialfi confronted him in his chambers. Knowing that this would be the only way to defeat the Lord of Asgard, Thialfi had only one chance to end his tyrannical reign. Thor called him a traitor, but Thialfi claimed this act to be one of loyalty to the Thor he once knew. During their conflict, the Enchantress, Thor’s wife, interfered and was met with a hostile reaction by Thialfi who viewed her as a sycophant that only encouraged Thor’s misdeeds. Unfortunately for Thialfi, even on the edge of victory, the Enchantress unleashed Thor’s pet wolf on him, and a surprised Thialfi was mauled by the beast thus ending his life.
* Note: Thor traveled back in time before the destruction of Asgard to make sure the events of this future would not come to pass, so it is possible that Thialfi did not fall into Zarrko's portal and emerge in the future. Even if this were true, Thialfi surely met his end, as did the rest of the Asgardians, during Ragnarok, but with the reappearance of Thor and the rebuilding of Asgard, it has yet to be revealed if Thialfi will stand at Thor's side once again during Thor’s quest to find his people.