Actor Tom Hiddleston plays Loki in the Avengers films. Image by Alberto E. Rodriguez/WireImage/Getty Images
In Norse mythology, Loki is known as a trickster. He is described in the Prose Edda as a "contriver of fraud." It's important to remember that "trickster" does not mean someone who plays fun jokes and pranks - Loki's trickery is all about mischief and mayhem.
Origins and History
Although he doesn’t appear often in the Eddas, Loki is generally described as a member of the family of Odin.
There is little archaeological reference to Loki (pronounced LOW-key), but in the small village of Kirkby Stephen, England, there is a tenth-century stone with a carving on it.
It is believed that the bound, horned figure carved upon the stone is in fact Loki, who was likely brought to England by Saxon settlers in the area. Also, near Snaptun, Denmark, there is a stone from around the same time as the Kirkby Stephen stone; the carving on this one is identified as Loki as well, due to scarring on the lips. In a story in which he tries to get the better of the dwarf Brokkr, Loki is disfigured and earns the nickname Scar-lip.
Although some Norse deities are often associated with symbols - such as Odin and his ravens, or Thor and his mighty hammer - Loki does not appear to have a particular item assigned to him by the Norse eddas or sagas. While there has been some speculation that he may be associated with particular runes, there is no scholarly or academic evidence to support this. Furthermore, this is an illogical argument in the context of Norse culture - keep in mind that stories and legends were passed down orally, from one generation to the next, and not written down.
As to his physical appearance, Loki was a shapeshifter and could appear any way he liked. In the Gylfaginning, which is one of the Prose eddas, he is described as being "pleasing and handsome, " but there are no details as to what those words describe.
Early carvings portray him with horns on his head, but those may be a representation of one of the shapes he adopts, rather than his regular form.
A shapeshifter who could appear as any animal, or as a person of either sex, Loki was constantly meddling in the affairs of others, mostly for his own amusement. Disguised as a woman, Loki fools Frigga into telling him about the weakness of her son Baldur. Just for fun, Loki tricks Baldur's blind twin, Hod, into killing him with a spear made of mistletoe. At one point, Loki spent eight years disguised as a milkmaid, and got stuck milking cows because his disguise was so convincing.
Loki is typically described as the husband of the goddess Sigyn, but he seems to have procreated with just about anyone and anything that struck his fancy. Because he could take male or female form, at one point Loki turned himself into a mare and mated with a mighty stallion, so he actually was the mother of Odin's magical eight-legged horse Sleipnir.
Loki is known for bringing about chaos and discord, but by challenging the gods, he also brings about change. Without Loki's influence, the gods may become complacent, so Loki does actually serve a worthwhile purpose, much as Coyote does in the Native American tales, or Anansi the spider in West African lore.
Worship & Celebration
Despite his divine or demi-god status, there's little evidence to show that Loki had a following of worshipers of his own; in other words, his job was mostly to make trouble for other gods, men, and the rest of the world.
For an excellent dissertation looking at Loki in his many forms, read Shawn Christopher Krause-Loner's paper Scar-lip, Sky-walker, and Mischief-Monger: The Norse God Loki as Trickster.
Honoring Loki Today
Loki has seen a resurgence in interest lately, due in no small part to his portrayal by actor Tom Hiddleston (see photo above) in the Avengers films, but just because he's becoming popular doesn't mean it's a good idea to call upon him. If you've spent any time reading Norse mythology, you know that Loki is a bit of an outcast, slightly manic, will do sneaky things for his own amusement, and doesn't seem to have much respect for boundaries.
If you invite Loki into your life, there's a possibility you won't be getting rid of him until he's good and ready to leave.