A prince who finds out he was taken as a war trophy rather than born to the family that raised him, and who spends the rest of his life proving the point badly.
Loki is the second son of Odin and the only one who knew, growing up, that something was wrong. He is not Asgardian. He was left to die on Jotunheim, the infant son of the Frost Giant king, and Odin took him home and changed his skin with magic without ever telling him. He finds out in the middle of an argument, which is roughly the worst possible way.
Everything he does for the next several films is an attempt to prove that the version of himself he was handed is a lie, and he does it by being exactly the person Asgard already suspected he was. He is a liar in a family of warriors, which he understands as an insult and the story understands as a job.
What keeps him from being simply a villain is that his methods keep failing on their own terms. He takes the throne twice and does nothing with it. He conquers a city and loses it in an afternoon. The character only becomes interesting once he is finally shown the whole record of his own life and made to sit through it.
Show detailsD. B. Cooper
Among the things attributed to Loki before the films begin is the D. B. Cooper hijacking. On 24 November 1971 a man travelling under the name Dan Cooper took Northwest Orient Airlines Flight 305, a Boeing 727 flying Portland to Seattle, told the crew he had a bomb, collected two hundred thousand dollars and four parachutes, and jumped out of the aircraft over southwest Washington. He was never identified and never found.
The joke is that this material will take a genuinely unresolved piece of history and hang a god on it, in the same series that asks to be read as a coherent record. It is worth knowing which parts of the timeline are load bearing and which are somebody having fun.
Major spoiler
Thanos kills him in the first ten minutes, by hand, in front of Thor, after Loki hands over the Tesseract to buy his brother’s life and then tries one last trick.
This is the reason the Loki of the series is the 2012 version and not this one: that Loki’s story is finished, and the show is about a different copy of him who never lived through any of it.