Heimdallr, or Heimdall as he is more commonly addressed, is one of the deities in Norse Mythology, called the "the whitest of the gods" in the Edda. Heimdall is the guardian of the Bifrost Bridge, or the rainbow, and therefore the link between Midgard and Asgard.
Legends foretell that he will sound the Gjallahorn, alerting the gods to the onset of Ragnarok, where the world ends and is reborn. Heimdall was destined to be the last of the gods to perish at Ragnarok when he and Loki would slay one another.
Heimdall, as guardian, is described as being able to hear grass growing and single leaves falling, able to see the end of the world, and so alert that he requires no sleep at all. Heimdall is described as a son of Odin, perhaps a foster son.