Loki, the God of Mischief aka the God of Evil in Norse mythology.
Loki is a Norse trickster-god who evolved from a troublesome prankster to an ever-more malicious entity throughout the many stories of Norse lore until he was ultimately punished by the gods for his treachery by being bound to a rock until the day of Ragnarok while a serpent drips venom into his eyes from above.
Perhaps unsurprisingly, Loki is said to lead the evil forces of Ragnarok once freed in an apocalyptic war against the gods, according to most legends during this battle he and the god Heimdallr shall face in combat and slay each other.
In the Danish comic series and the movie Valhalla which is remarkably close to the myths, he is a considerably comedic and harmless con-man. While boastful and rude whenever he gets the chance he remains a coward at heart ready to slip away from any actual danger.
Appearances In Modern Media
Loki is one of the most infamous of the Norse gods and has been described in many films, comics, videogames and books as well as traditional lore - his monstrous children are equally famous in the form of the giant wolf-monster Fenrir, the world-serpent Jormugandr and last his own daughter, the Norse goddess of the underworld Hel.
Some of the most famous incarnations of Loki in modern fiction has been the supervillain in Marvel Comics, in which he is basically a slightly updated and toned-down version of the trickster god of folklore and the villainious Loki from the Son Of Mask - who was considerably less like his mythical counterpart and more in line with a run-of-the-mill evil magician/prankster.
Loki has also appeared in the Stargate series - in which he was a rogue Asgardian conducting illegal experiments on humans - this version of Loki is even less like the mythological one than the version in Son Of Mask and is in effect an archetypical alien invader akin to the Greys seen in series such as the X-Files.
Other incarnations of Loki and characters inspired by him have appeared in innumerable works of fiction - probably too many to list here, these are just a sample of the few that are aware of by all.
Powers
Loki is depicted with many different powers at his disposal, in fiction and in folklore, shape-shifting seeming to be one of his specialities in the old tales while, as a god, it is not unreasonable to assume he could pretty much do anything he desired - though he would not have anything like the power of a god such as Odin - unusual for a Norse god Loki was said to have a large degree of command over monstrous creatures such as trolls and giants, traditional enemies of the gods - though this could be down to the fact Loki himself is sometimes considered a half-giant and is the father of a few notable monsters himself. Also, while not on Thor's level, Loki seems to have a enormous degree of superhuman strength, causing earthquakes every time he writhes in pain.