Loki Laufeyson is the Asgardian God of Mischief, currently exiled to the Earth-Realm. He plans to enjoy the stay.
In the distant realm of Asgard, a war raged for centuries between the Asgardians, lead by their King Odin, and the Frost Giants of Laufey. Eventually Laufey was slain by Odin, only to leave behind a very unusual prize: a babe, resembling an Asgardian, but assumed to be Laufey's own. Touched and having a twisted sense of 'honoring' a noble opponent, Odin adopted the boy as his own, naming him Loki. This would turn out to be among Odin's numerous mistakes.
From a very early age, it was clear that Loki was different from other Asgardians; he wasn't as strong or possessed as strong a calling to the battlefield, especially when compared to his adopted brother Thor. He much preffered manipulating magical energy and playing tricks on people, Thor being among his favorite targets. A sibling rivalry for Odin's affection was in full-swing by the young princes' adolescence, each vying to be the next king of Asgard. While Odin tried to not play favorites, his clear favortism for his actual biological son was no secret. And that whole prophecy of Loki being the bringer of Ragnarok, the end of Asgard and all reality, seriously hurt his PR.
Once arriving in the Earthly realm, Loki discovered, much to his surprise, that beings existed here even more powerful than the strongest Asgardian warriors. He slowly began to form a plan, manipulating weak-willed Earth creatures to his sinister and selfish will, and occasionally merely causing mischief for the sheer pleasure of it. Of course, his brother Thor wasn't far behind, making his life more difficult. But Loki always welcomed the chance to battle wits with his stronger, blonder brother.
Frustrated and convinced that he would never be given his rightful role of King of Asgard, Loki began to put motions into place to simply take the position by force. Of course, being weaker and less skilled in combat than most Asgardians, he had to turn to his skills of deceit and mischief, attempting to turn the public opinion against the 'senile and half-blind king'. He even allied himself with the cruel Frost Giants, proving to them that he was the Laufeyson, the rightful heir to their throne as well. Eventually, however, Thor would lead the charge against Loki's plans, exposing him and leading him to be banished from Asgard. Defeated and ashamed, he fled to the realm of Earth to regroup.
Recently, Loki has aligned himself with other villains, offering his services in hopes of earning allies in his goal of invading and ruling Asgard. He has convinced himself that the prophecy of Ragnarok is not a warning, but a promise of things to come. The end of Odin's Asgard, and the beginning of Loki's. And if he can have some fun making a few mortals miserable along the way, all the better.
Loki can seem friendly and helpful when he wants to. Don't fall for it. The truth is that few people are as cruel, selfish and deceitful as the God of Mischief. He doesn't see other people so much as he see tools, and he certainly has no real sympathy for anyone. His every move, his every word is somehow designed for his own benefit, and any piece of it that might seem altruistic is merely a feint. Put quite simply, he doesn't care about anyone else, and for the most part hates other people.
Of course, that doesn't mean he can't ACT like he likes other people. Loki has the ability to seem charming, friendly and even caring if it suits him, though most of these are mere masks. The only time his emotions are truly out of his control is when someone questions his power or righteousness, having a short temper for being questioned or belittled.
Part of the reason for his pride and viciousness is a deeply-seeded sense of victimization. He never felt at home among the Asgardians, in large part because they never fully accepted him as one of their own, but almost as an novel interest or even a pet. Odin called himself Loki's father, but still treated him far beneath Thor in his favor. His sense of worthlessness early in life fueled his need to feel victorious.
Finally, Loki enjoys a challenge. He appreciates a worthy opponent and only seeks out the gifted in one way or another as Pawns, attempting to overcome their intelligence or willpower to use them to his own goals. Little or unexceptional people bore him, and he often ignores or simply dispatches with them. His elitism has gotten him in trouble before, but by and large, what can the boring and sad do to him, the True Heir to the Throne of Odin?
While not a proper Asgardian, Loki does come from the magical realm. As such, he has a superhuman physiology compared to most humans. This includes strength capable of lifting 50 tons, an accelerated agility capable of pushing him to full capacity for 24 hours before tiring and durable enough to shake off bullet wounds and falls from extreme heights. His body is also three times as dense as standard humans, and he ages much slower, capable of living for thousands of years.
Loki is also an accomplished sorcerer, capable of enhancing a creatures strength, including his own, cause accelerated healing up to reattaching his own head to his body (if he moves quickly), giving strong telepathic commands and bringing inanimate objects to life. He is also able to teleport across dimensions, making access to Asgard relatively easy despite that pesky ban. Spells must be cast by channeling magical energies, which the Earth-realm is notorious for lacking comparatively to Asgard, and then maintaining concentration on the spell. If his concentration is broken, or there is not enough magical energy to maintain his hold, his sorcery will be interrupted and Loki will be very cranky.
Finally, Loki is an accomplished shapeshifter, able to completely change his appearance to nearly any form, human or animal. His density cannot be increased, meaning his weight constant. He also finds shape-shifting to be taxing, physically and mentally, and can start to lose his grip on his sanity if in an unfamiliar form for too long.
Loki is a master deceiver and the self-declared God of Mischief, though few people would argue with this claim. His magical abilities give him insight into people's desires, and he uses those desires to turn people into his willing puppets, making promises that he intends to twist around to better his sinister purposes. He is an excellent judge of character, charismatic and charming when he needs to be, and threatening and menacing the rest of the time.
In addition to his skills in manipulation, Loki is a master magician, with great insight into the occult realm. He has an expert grasp on Asgardian lore and history, which itself is filled with magical wonders. He can recognize and manipulate magical objects with the same comfort that one might operate a remote control.
While Loki is not as valiant or strong as most Asgardian warriors, he is still fascinated by battle, specifically tactics and strategy. He has a keen mind for crafting battle strategies, personified by his obsession with the Earth-realm game chess. He is also a skilled swordsman, preferring finesse to pure overpowering.
Loki has created a small but loyal network of followers, whom he refers to as his Pawns, that will do his bidding in return for him doing them favors in return. He has declared himself the king of the Frost Giants in Asgard, a title that the Frost Giants mostly respect do to Loki's lineage. He also is a member in standing with the Masters of Evil, mostly due to his own personal interests, attempting to win their favor to in turn enlist them in storming Asgard. He also is the current owner of the mythical sword Laevateinn, a magical weapon native to Asgard capable of seriously wounding even the gods of that Realm.
Loki has a difficult time making friends. Pawns and assistants, easily, even war partners, but eventually his mischievous and treacherous ways make him a pariah to all that he might consider a long-term ally or confidant. His history of trickery has created quite a reputation for himself, and anyone who knows the first thing about him knows he can't be trusted. This social pariah effect most dramatically got him banned from his home-realm of Asgard, stranding him on the far less cool Earth-realm to plot his revenge and eventual triumph.
Not that Loki minds much not having friends, even if it does make his ultimate goals trickier. He doesn't trust anyone else anyway, believing he is better off on his own, above the world. He is an egotist if there ever was one, with his pride sometimes blinding him to his flaws; if a Pawn questions him, he won't take it well, and often as a point of pride will purposefully not fix the issues they suggest...even if their suggestions are perfectly reasonable. This hubris has been his undoing more than once.
As mentioned before, Loki makes his home primarily in the Earth-realm, venturing back to the Asgard only in disguise or hiding. As such, drawing upon magical energies can be extremely difficult, meaning that his magical abilities are severely limited. They require his full concentration to remain active, and even then he will likely become drained after only a few hours of consistent use, having drawn all the magical energy in his area. Also, his most useful of magic abilities, the ability to change his form, is also among the most painful. Staying in any form too long causes him great physical and mental anguish, sometimes causing him to lose bits of his memory or even his identity; he spent a couple decades as a woman, only because he didn't realize that he wasn't originally a man.
And then there is Thor, Loki's brave, blonde and bully adopted brother. Thor is a big jerk, who always gets in Loki's way and make things generally more difficult. And all because Thor thinks he's the rightful heir to Odin's throne. To make matters worth, Thor occasionally enlists the help of his hero friends to stop Loki's plans. Again, total bully.