"Serving-elf?" The servant of Thor, and the brother of Röskva. He is the swiftest-footed of all men. In Gylfaginning, Snorri Sturluson tells how Thor acquired his servants:
Öku-Thor drove forth with his he-goats and chariot, and with him that Ás called Loki; they came at evening to a husbandman's, and there received a night's lodging. About evening, Thor took his he-goats and slaughtered them both; after that they were flayed and borne to the caldron. When the cooking was done, then Thor and his companion sat down to supper. Thor invited to meat with him the husbandman and his wife, and their children: the husbandman's son was called Thjálfi, and the daughter Röskva. Then Thor laid the goat-hides farther away from the fire, and said that the husbandman and his servants should cast the bones on the goat-hides. Thjálfi, the husbandman's son, was holding a thigh-bone of the goat, and split it with his knife and broke it for the marrow.
Thor tarried there overnight; and in the interval before day he rose up and clothed himself, took the hammer Mjöllnir, swung it up, and hallowed the goat-hides; straightway the he-goats rose up, and then one of them was lame in a hind leg. Thor discovered this, and declared that the husbandman or his household could not have dealt wisely with the bones of the goat: be knew that the thighbone was broken. There is no need to make a long story of it; all may know how frightened the husbandman must have been when he saw how Thor let his brows sink down before his eyes; but when he looked at the eyes, then it seemed to him that he must fall down before their glances alone. Thor clenched his hands on the hammer-shaft so that the knuckles whitened; and the husbandman and all his household did what was to be expected: they cried out lustily, prayed for peace, offered in recompense all that they had. But when he saw their terror, then the fury departed from him, and he became appeased, and took of them in atonement their children, Thjálfi and Röskva, who then became his bond-servants; and they follow him ever since.
The siblings accompanied Thor and Loki to Útgarða-Loki's keep, where their party was subjected to a series of tests. When asked what Þjálfi could do, he answered that he would undertake to run a race with whomsoever Útgarða-Loki would bring up. The giant said that that was a good accomplishment, but he would like to see this put to the test. He called to him a certain lad, who was named Hugi, and bade him to run a match against Þjálfi. Hugi convincingly won three times out of three races. The giant complimented Þjálfi on his speed, saying that never have any men come to Jötunheimr who were fleeter of foot. It is later revealed that Þjálfi raced against thought itself, which outpaces all.
In another story, Thor was challenged to a duel at Grjótúnagard by the giant Hrungnir, and Þjálfi joined him as his second. Hrungnir appeared with a huge clay giant named Mökkurkálfi. In the ensuing fight, Thor dispatched Hrungnir with his hammer while Þjálfi attacked Mökkurkálfi, and the clay giant fell with little glory.
In Þórsdrápa he accompanied Thor to the home of the giant Geirröðr. His bravery is attested in a stanza from the poem:
- Wroth stood Röskva's Brother,
- And Magni's Sire wrought bravely:
- With terror Thor's staunch heart-stone
- Trembled not, nor Thjálfi's.
- Gylfaginning, 44-45, 46.
- Skáldskaparmál, 4.