Loki Laufeyson is one of the oldest villains in the Marvel Universe. As a centuries-old Norse God of mischief, he is the first villain that the Avengers ever faced. His presence has been a constant nuisance for both the Avengers and his brother Thor, the God of Thunder. His consistent appearance and importance in the Marvel Universe also led him to appear in the MCU, played by Tom Hiddleston. However, while a majority of his life was spent in his original form, recent events led to the emergence of a few other incarnations for the God of Mischief.
Classic Loki
Loki's longest-running, and most recognizable incarnation, is the classic God of Mischief Loki. This was Loki's form until he died in Brian Michael Bendis and Olivier Coipel's Siege and was reincarnated as Kid Loki. In this form, Loki was mostly villainous, relying on mischief to cause trouble for Thor and the other heroes of the Marvel Universe, doing so simply for the fun of it. In this form, Loki primarily wore a green full-body leotard, with his classic gold long-horned helmet. Loki died in the comic after pulling his patented backstabbing and mischief on Norman Osbourn and The Hood, leading them and the Void to attack Asgard.
However, after Norman Osborn has the Void destroy Asgard, Loki realized he had gone too far and prayed to Odin, then using the Norn stones to aid the Avengers before the Void killed him. Loki's last words were "I'm sorry, Brother. However, after having his sister Hela erase him from the book of Hel, untying him to Hel or Asgard and granting him absolute freedom, thus allowing him to be reincarnated as Kid Loki, though he would never be the original Loki.
Kid Loki
After breaking himself free from the chains of destiny by having Hela remove his name from the Book of Hel Loki was reincarnated. Thor found Kid Loki in Paris where he was living as a street hustler and had no memory of who he was. The God of Thunder would tell Kid Loki who he was, and despite initially thinking Thor was crazy, he would start to believe him as he was having memories of his past life. Thor was eventually able to restore part of Loki's identity, though he remained a child. However, he was still unwelcome in Asgard and was scared off by Odin.
Despite their distrust of him, Loki still attempted to help Thor protect Asgard, though he pulled a trick that Thor was unaware of leading the God of Thunder to threaten his brother similar to their father. Loki would later meet an echo of his former self who would try and convince him to pick up the plan he had set up in his death, though the younger Loki wanted no part of this as he wanted to be his person. He would then dismiss his elder self and turn him into a magpie, then condemning him to eternity as Ikol, his minion and opposite.
Ikol
After his creation at the hands of Kid Loki, Ikol found himself displeased with how his new and younger incarnation was acting. He began meddling in the Younger Loki's attempts to turn over a new leaf, before finally revealing he only existed in Kid Loki's mind during a mission to stop Mephisto from getting the Fear Crown. Ikol presented Kid Loki with the only option to destroy the Fear Crown. Being that the fear clot that made up the crown had come from Kid Loki's mind if Kid Loki's mind was destroyed, so would the crown.
Ikol also revealed the only way for Kid Loki to die was to let Ikol overwrite his mind, take his body, and make Kid Loki's actions his own. Kid Loki accepted his fate, though not before taunting Ikol and making it known that Ikol had already lost by doing this. Ikol acknowledged his defeat and took over Kid Loki, rewriting his body and becoming the only Loki.