Among all the gods, giants and dwarves there are two who are different: Thjálfi and Röskva, a pair of farmer's children – the only humans who travel permanently in the retinue of a god. How they came to be there is told in one of the loveliest little stories about Thor.
Thor rides in a wagon drawn by two goats – Tanngnjóstr („tooth-gnasher“) and Tanngrisnir („tooth-barer“). Their wonder: Thor can slaughter and eat them – if he then lays the hides and bones back together unbroken and hallows them with Mjöllnir, the goats rise alive again the next morning.
On a journey Thor stops at a poor farming family and shares his goats as a meal. But the son Thjálfi secretly splits a leg-bone to get at the marrow. In the morning one of the goats rises up lame. Thor's wrath is tremendous – the family fear for their lives. In atonement they give him their two children, Thjálfi and Röskva, as servants. Ever since, the siblings accompany the god of thunder.
Thjálfi is no mere farmhand. He is reckoned the fastest runner of all and races in Utgard (against „Hugi“, thought). At Thor's duel with the giant Hrungnir, Thjálfi slays the giant's helper, the clay giant Mökkurkálfi. The farmer's son thus stands for humankind at the side of the gods – capable, brave, indispensable.

Thor (Donar) · Thor at Utgard-Loki · The Midgard Serpent · The Giants.
Poetic Edda: Hárbarðsljóð. Snorri Sturluson, Prose Edda (Gylfaginning: the goats, Thjálfi & Röskva; Skáldskaparmál: Hrungnir, Mökkurkálfi), c. 1220. German public-domain translation: Karl Simrock, Die Edda (1851). Scholarship: Rudolf Simek, Dictionary of Northern Mythology; John Lindow, Norse Mythology (2001).