Heimdall is the ever-watchful sentinel of the gods, stationed at Bifröst, the burning rainbow bridge that joins Asgard to the world below, so that no giant may cross unseen. He is said to need less sleep than a bird, to see clearly a hundred leagues in every direction by day or night, and to hear grass growing in the meadow and wool growing on a sheep’s back. His teeth are gold, and he is called, oddly among the gods, the son of nine mothers — the wave-maidens, daughters of the sea giant Ægir — which ties him as much to the ocean’s edge as to the sky’s.
Heimdall keeps the horn Gjallarhorn ready at all times; when Ragnarök finally comes, its blast will wake every realm to the last battle. He and Loki are old enemies, having already fought once, as seals, over the theft of Freyja’s necklace Brísingamen — a rivalry that is settled for good at the end of the world.
Loki á orrostu við Heimdall, ok verðr hvárr annars bani
“Loki shall have battle with Heimdallr, and each be the slayer of the other.” — Gylfaginning 51, tr. Brodeur
Little narrative survives of Heimdall compared to Thor or Odin, but his role as threshold-guardian, poised between worlds and utterly incorruptible, is constant throughout the sources.