Marvel's Loki Laufeyson first appeared in Venus #6 in 1949 and resurfaced in Journey into Mystery #86 in 1962. Loki is the adopted son of Odin, adoptive brother/archenemy of Thor, and the "God of Mischief".  His name is a reference to the Norse God Loki. 

Bor, once king of Asgard, was tricked during a battle with Frost Giants and turned into snow. Odin, Bor's son, found him but made no attempt to save his father. Odin, instead, decided to run Asgard himself- but was cursed by Bor for refusing to help. Not a week later Odin found himself at war, once again, with the Frost Giants. Odin slays Laufey in hand to hand combat and finds his son, Loki, nearby. Odin adopts Loki because he is the son of a worthy adversary slain in combat. 

Loki grows up with Odin's real son, Thor, and is resentful of how different they are treated. The people of Asgard saw in Thor things they valued like size, strength, and bravery. In Loki they saw only cleverness and mischief- and they did not appreciate that Loki was growing into a powerful sorcerer. Loki played tricks on Asgardians and earned himself the title of "God of Lies and Mischief". 

As Loki grew into adulthood his trickery turned more malicious and he was imprisoned for his actions. He escaped from Odin's sentence and sought training from the sorcerer Eldred. Eldred taught Loki the dark arts until Loki was his magical equal. 

...but Loki was ravenous for more power.

Loki sought out the fire demon Surtur. Loki offered Eldred's soul to Surtur under the stipulation that he (Loki) could absorb Eldred's power and maintain control of his lands, possessions, and followers. Surtur and Loki tricked Eldred and finalized their deal. Loki's actions earned him the title of "God of Evil". 

Loki appears in comics and movies to this day in a constant search for more power. His ambitions in recent installments have been to control the 9 realms but, historically, Loki really just likes ruining things for brother Thor.