Loki Laufeyson

Loki is the adopted brother of Thor, the adopted son of Odin and Frigga and the biological son of Laufey and the enemy of The Avengers.

Thor
Adopted by Odin and Frigga at the end of the Asgardians' war with the jotuns, Loki was treated as a prince of Asgard and never informed of his true heritage throughout his youth. Jealous of his of step-brother Thor, Loki allowed several Jotuns to enter Asgard to try and steal the Casket of Ancient Winters during Thor's coronation. In the wake of this intrusion, Thor opted to launch a counterattack on Jotunheim with a number of his friends, and while Loki initially tried to dissuade his brother, he eventually relented and went along with the plan.

Once on Jotunheim, Thor again led them into conflict and attacked the Jotuns even after being granted safe passage home. During the ensuing fight, Loki was surprised by his own resistance to the Jotuns' freezing touch, a revelation that led him to conclude that he was at least partly Jotun rather than an Asgardian as he had been led to believe.

Loki confronted Odin over his lineage, but gained little more than an historical account of his early life before Odin lapsed into his "Odinsleep". With Thor already banished to Earth for the attack on Jotunheim, Loki stood unopposed and assumed the role of Asgard's regent. From his position of power, he continued to manipulate others, firstly making Thor believe Odin had died, then forcing obedience from other Asgardians and finally manipulating the Jotuns into making an attempt on Odin's life.

When Thor's friends disobeyed Loki to bring word of his actions to Thor, Loki responded by sending The Destroyer after them to eliminate Thor. This action proved his undoing when Thor's courage in confronting the automaton paved the way for his return, and just as Loki revealed his double-cross against the Jotuns who were infiltrating Asgard, Thor arrived to challenge him over his actions. The resulting fight quickly moved to Heimdall's Observatory where Loki had opened a portal that would destroy Jotunheim. Thor's power ultimately overpowered Loki's cunning, at the cost of the observatory and the Bifrost.

As Loki fell into the abyss below, the spear in his hand was grabbed by Thor who in turn was anchored to the remnants of the bridge by the newly-awakened Odin. He appealed to his stepfather that by annihilating the Jotuns he was doing what was best for Asgard. Odin sadly rejected this, and with his Asgardian life now in tatters Loki released the spear and allowed the void to claim him.

The trickster was not so easily felled however, and found his way to Earth where he began to influence the human population again. One in particular, Erik Selvig, who was summoned by S.H.I.E.L.D. to turn his scientific knowledge to the task of unlocking the secrets of a mysterious cube. Loki, as an apparition in control of the man, whispered words of encouragement to proceed.

The Avengers
To be added

Character traits
Unlike his stepbrother, Loki is not an impulsive man, quite the opposite: thoughtful, manipulating, farseeing, he's the perfect tactician. Loki is mischievous by nature, always eager to prove himself: being physically weak, he developed a sharp intellect and an acute mind to overcome his handicap amond the Asgardians warriors. He's pitiless and obstinate, ready to destroy everything on the path to his aims. Brilliant and smart, he's an incredible foe for a society mostly based on mindless combatants.

Being a misshapen Frost Giant, Loki possesses some of the characteristics of his kind, as the resistance to extreme cold and to his brethrens's frost powers. He also possesses superhuman physical abilities, such as strength, stamina, speed and reflexes, but to an extend equal or even near to the Asgardians. He's a skilled magician, specialized in the art of illusion, and can create mirages and false images of himself, make himself invisible to anyone, included the omniscient Heimdall, and can even control the minds of weaker beings as the humans. Loki proves some skills in combat wielding Odin's spear, Gungnir.

Powers

 * Sorcery: Loki can create illusions, including copies of himself. He can influence people with his mind. He can shrouded himself or others. In the movie, Loki the Frost Giants from Heimdall. Loki can summons objects out of thin air. In the movie, He summons The Casket of Ancient Winters and used it against Heimdall freezing him solid.
 * Teleport: Loki can travel to other dimensions without the Bifrost. The full limits of this abilitie aren't known.

Friends and Allies

 * The Destroyer
 * Erik Selvig - Unwitting Pawn

Enemies

 * Thor - Step-brother
 * Odin - Adopted Father
 * Frigga - Adopted Mother
 * Sif
 * Laufey - Biological Father
 * Warriors Three
 * Frost Giants
 * The Avengers

Behind the scenes

 * At first, Tom Hiddleston auditioned to play Thor, but Kenneth Branagh decided he was fitter to play Loki.
 * Hiddleston had to go through a strict diet since Branagh wanted Loki "to have a lean and hungry look".
 * Hiddleston took inspiration from Peter O'Toole's interpretations from The Lion in Winter and Lawrence of Arabia.

Appearances/Actors

 * Canon (2 films)
 * Thor (First appearance) - Ted Allpress, Tom Hiddleston
 * The Avengers - Tom Hiddleston

Trivia

 * Loki is based on the god of mischief in Norse mythology.
 * In the comics, Loki never meets his biological father after being adopted by Odin, while in the movie he even allies (and betrays) him.
 * At the end of Thor, Loki looking at the Cosmic Cube and deceiving Nick Fury is a hint to the comics continuity, in which he's the villain to stop which The Avengers assembled for the first time.