Half-brother to Thor and son of Odin. What little of Loki's past can only be gleamed from brief statements by Thor and himself. Hailing form Asgard, Loki was born to the Norse god-king Odin. Growing up in Asgard, Loki became known as the god of mischief for his pranks and trickster nature. As time went on he became increasingly jealous of the favoritism his father Odin showed to Thor. Loki's resentment caused him to use his powers over reality to make Thor's life as difficult as possible, but he always stopped short of outright killing Thor due to fear of their father. Over the centuries Thor battled Loki several times and was always able to stop his schemes or undo the damage he did. Eventually Loki went too far and was imprisoned in the Room Without Doors by Odin three hundred years ago.
Following the failed Chitauri invasion of the world, Loki escaped his prison and made his way to Earth to take revenge against Thor. He conspired with the Liberators to destroy the Ultimates and start World War III per his role as god of mischief. Using his powers over reality, Thor created the identity of Gunnar Golmen, the Norwegian super-soldier program and Thor's past life as Thorlief to make Thor appear insane and turn the Ultimates against him. He also used his powers to frame Captain America for the murder of Hawkeye's family. His plans culminated in the invasion of the United States, where he participated as one of the leaders. Throughout this, he hesitated from using his powers openly, since doing so would enable his father to find him.
Despite the careful planning of the Liberators and Loki, the Ultimates were able to turn the tables and kill most of the Liberators and their soldiers. Faced with little alternative, Loki decided to take matters into his own hands to destroy the Ultimates himself. Before he could, the Scarlet Witch used her probability-powers to summon Loki's brother Thor.
Faced with his angry brother, Loki attempted to trap him in an illusion, but for unknown reasons, according to Thor, Loki's powers had begun to weaken. After surviving uninjured a direct assault from Thor, Loki decided to simply kill Thor and all the Ultimates by summoning an army of Asgardian monsters, while battling Thor himself. At this point, Loki had lost all fear of Odin and was on the verge of killing Thor before Odin intervened. Odin sent an army of warriors of Asgard to battle the monsters and removed most of Loki's powers, making him vulnerable to Thor's hammer. Without his powers, Loki pleaded with Thor and ranted how he had been merely jealous of Thor and wanted to see him fail. He also pointed out the absurdness of his manipulations and the humor in humans failing to realize it. Thor, not impressed, summoned a huge lighting bolt and banished Loki back to Asgard for punishment from Odin.
Loki has the ability to "shuffle time and space" as described by Thor. He can retcon history into anything he desires by either creating events that never happened, such as his mortal identity, or erasing events as if they never happened. He can also shape-shift, render himself invisible, transverse dimensions, fly, summon various creatures, alter the color of the sky, hide from the sight of other gods, and make himself impervious to blows from Thor's magic hammer. The only known limit on his powers are his imagination and limits placed on him by Odin.
Loki is a master schemer.
Loki's strength is at least equivalent to Thor's when there are multiple copies of him, other wise it can be assumed that he is weaker than Thor physically.
Loki is bound by limitations place on him from Odin preventing him from outright using his powers to kill Thor or humans, he seemed to temporarily be able to use his powers at full capacity but it can be assumed that the full use of his power led Odin to be aware of his location and he started to lose his powers.
- Thor mentioned in their fight that Loki's powers for an unknown reason had weakened. Various comments made by Loki throughout Ultimates 2 implied Odin prevented Loki from using his full power and killing Thor.
- Loki is currently one of if not the most powerful known character in the Ultimate Universe (although it can be presumed that Odin is much more powerful due to the control he has over Loki and the fear Loki has of Odin finding him).