Loki

 

“I, Loki, Prince of Asgard... Odinson, the rightful King of Jotunheim, God of Mischief, do hereby pledge to you, my undying fidelity.”

―Loki to Thanos just before attacking him to save Thor

Loki, born Loki Laufeyson and also known by his adoptive name Loki Odinson, is a supervillain turned hero based on the Norse God of Mischief. He is a Marvel Comics character and one of the main characters in the Marvel Cinematic Universe. 

He is Thor Odinson's adoptive younger brother. This version of the mythological Loki was created by Stan Lee, Jack Kirby, Larry Lieber, Don Rico, Christopher Rule, Violet Barclay, Chu Hing, Mike Sekowsky, Pierce Rice and Syd Shores.

Introduced as the main antagonist of both Thor and The Avengers, he later allies with his brother after his mother's death during the events of Thor: The Dark World. 

In Thor: Ragnarok, he joins the Revengers to save the people of Asgard from his evil adoptive sister Hela and from the world's imminent destruction. Finally recognizing himself as a true son of Odin, Loki eventually sacrifices himself in a heroic act to save Thor from Thanos in Avengers: Infinity War.

In Avengers: Endgame, however, while the Avengers time-traveled to get the Infinity Stones from the past, Loki from an alternate timeline manages to steal the Tesseract and escape both from being arrested and from his eventual predetermined fate, leading him to be the protagonist of the Disney+ Loki series.

 The series also introduces other alternate versions of Loki, the main one being a woman that renamed herself Sylvie, who is a fugitive Variant being hunted by the Time Variance Authority and that becomes romantically involved with the protagonist.