Author notes: This is under the assumption that Ragnarok is a cycle of death and birth for the gods. There are several more well-known incarnations of Loki in this fic. I apologise for inaccuracies, but all I know of Loki is from what little I know of mythology, the movie, and the wonders of internet. I'm pretty sure I will cringe at this later when I've finally read the comics.
1.
Loki is a red-haired trickster. The son of Farbauti and Laufey, the blood brother of Odin. He lives amongst the Aesir as one of them and yet not. His trickster ways do not endear him to these gods, but none of his deeds are done with malice, not even when he places the mistletoe in Hodr's hand.
He does not mean to kill Baldr, it just happened that his aim was impeccable and the arrow flew true. He keeps telling himself that even as he flees from Thor and his hunters, even when he is bound by the entrails of his son whilst a serpent drips poison into his eyes. Even through Ragnarok, as he falls with Heimdall, he refuses to believe that he had meant to kill Baldr the Beautiful with that innocent little mistletoe.
It was just a jest like any other, to prove the point that even the Aesir are not all-knowing or almighty, that no one could truly be vulnerable. It was their fault that they decided to toss dangerous objects at their beloved god in the first place. Really.
2.
Loki is dark-haired now. He is a Laufeyson and adopted son of Odin. He is bitter, resentful that his intellect and skill in sorcery is nothing more than mere amusement to these so-called gods. He watches with bitterness as Balder draws much praise for all that he does, especially from Thor, even though Balder never was raised as a prince. He might be an Odinson in blood, but Loki is more of an Odinson than Balder would ever be.
Balder is said to be invulnerable, but Loki finds his weakness eventually. With malicious glee as Hoder, poor blind Hoder, shoots the mistletoe arrow into perfect, "invulnerable" Balder.
He has no regrets, not even at the sight of Balder's body burning at the funeral pyre.
3.
There are mistletoe weapons forged for Loki's army as he marches into Asgard. He is absolutely giddy to be bringing Ragnarok about. Yes, Balder had returned, but Loki, clever, skillful Loki, is making Ragnarok happen without even needing the prophecy of Balder's death to be fulfilled. He grins, knowing the fate of all the gods as he breaches Asgard's defenses. Not even Balder will escape death this time and Loki will have the satisfaction of proving that he is better than all of them. Truly, these Asgardians should've known better than to scorn and belittle him.
4.
He is the dark-haired half-brother of Thor and Balder, son of Odin and Farbauti. He is skilled in sorcery and with arms, especially with the bow and arrow. He is taken for granted by Thor and teased by Balder for being "grim and boring". He takes it all in stride while harbouring a secret resentment. Balder is the best in combat. Thor is the favourite of their father.
Balder is just in the way, really, when he let those arrows fly. He shouldn't have been there to stop Loki from stealing the Norn stones. Loki actually feels remorse at that moment, because he fears what Odin will do to him when he finds out that Loki has killed his handsome half-brother. It is an unnecessary complication in his plans.
5.
Loki is raised believing himself to be an Odinson, full brother of Thor, son of Frigga and Odin. Balder is a dear friend of Thor's, as dear as Sif and the Warriors Three. He is younger than even Loki, but cherished by all. Loki finds himself unable to hate him, although he does enjoy playing special pranks on the younger god.
It isn't until Loki finds out that he has been lied to all his life that things start to change. Loki falls and falls from the Bifrost and lands on Midgard with new ideas, with new goals. He will have his revenge on Asgard, on Odin, on Thor. He does not care that his first attempt at terrorising and ruling Midgard is foiled by the Avengers, he can wait. He is patient.
He knows about Balder's true origins. He would be a fool not to see the resemblance between Balder and his parents. He seeks Balder out when he is on Midgard, mistletoe dagger in hand. If he cannot bring Midgard and Asgard down yet, he would make Thor and Odin suffer.
Balder does not see Loki. Loki has gotten far too adept at travelling in the shadows, at making no noise. He plunges the knife into Balder's back and grins. He does not hate Balder then either, but he will do anything and everything to make sure his so-called "family" suffers just as much as he had over the centuries. The weaker their emotions, the easier it will be when Ragnarok arrives.
+1
Loki is the son of Laufey, but small and delicate in features amongst the Jotnar. He has unusual dark hair and almost feminine beauty, but no one dares call him weak for he is clever in words and formidable in sorcery.
Balder first encounters the strange Jotun prince on a diplomatic visit on behalf of his father, Odin. He is charmed by the prince's words, but even moreso by the cool steel behind those blood red eyes. He invites Loki to visit Asgard.
The two become fast friends, despite their different interests. Loki and Balder are often found in Asgard playing tricks on Thor and his friends or on Jotunheim infuriating Loki's younger brothers.
When the two grow into young men, Odin and Laufey discuss a potential marriage, a unity between the two realms for prolonged peace.
On the day of their wedding, Balder gives Loki an ornamental dagger with a handle made of mistletoe. When Loki questions Balder about the odd gift later that night, Balder tells his longtime friend and new spouse that mistletoe is his sole weakness.
During a heated argument years later, Loki finds the very same dagger in his hand, poised to plunge into the heart of his irritating, obstinate husband. Balder catches Loki's arm before he can act and leans in to kiss those thin, angry blue lips. Loki drops the dagger.
That evening, Loki calls upon the fires of Muspelheim and destroys the dagger before Balder and all is well.