Heimdall

Heimdall the Guardian

This god was the guardian of Bifrost, the rainbow bridge that links Asgard with other planes of existence (a permanent dimensional rift open to Earth and a few other places. 
Heimdall and Loki are ancient enemies. Once the two competed against each other to win a powerful magical item, the Brisingamen. This started the resentment, but the treacherous Loki has given The Guardian countless reasons to hate him since then.. Their rivalry, it is said, will not be resolved until Ragnarok, when the two gods will fight and kill each other. 
Heimdall is a warrior god charged with guarding Asgard from invaders and unwanted dimensional travelers appearing at the dimensional nexus of the rainbow bridge . Note: All dimensional visitors appear at/on the rainbow bridge. This is one of Asgard's unique defenses.
Heimdall rarely leaves his post at the rainbow bridge, but is occasionally sent on special missions by Odin.