Loki

In Norse mythology, the giant-born god and blood-brother of Odin, companion of the Aesir (principal warrior gods), but a source of trickery and evil, and the cause of dissension among the gods. Instrumental in the slaying of Balder, he hastened the coming of Ragnarök, the final battle of the gods. His children by the giantess Angrboda were the Midgard serpent Jörmungander, which girdles the Earth; the wolf Fenris; and Hel, goddess of death.

Heimdall, sentinel of the gods, was immune to Loki's entertaining friendship with the Aesir, and became his chief enemy.