Baulder Odinson is a major supporting protagonist in the Kingdom Hearts Unlimited Saga, first appearing in "The Keyblade Wars" as the fabled third son of Odin in the Asgard campaign. He later returns in "Return of the Keyblade" in the Ragnarok campaign as a secret protagonist brought forth to perform his destiny to save Asgard or die trying.
According to Norse mythology, Balder was the brave Asgardian god of Light, and his death was foretold as both a great tragedy to the Aesir and a harbinger of Ragnarök. Although he was loved by all gods and more physical beings, he was so handsome, generous, and good that he gave off light simply by the purity of his character.
He was originally the youngest son of Odin after Tyr, Thor, Vidar, Vali, and Loki, but he died during the final battle of the Keyblade War in a valiant effort to try and save the One World of Legend from destruction.
When the current era began over a thousand years ago, Odin, Frigga, Cul, and Hela were the first four gods to be reincarnated with all of their memories intact of their past selves, with Odin being forced to call upon his brother and neice to serve as his executioners in his misguided attempt to restore order to the 9 Realms in the wake of the Keyblade War's aftermath by conquering them all. Odin would soon realize through remembrance due to Frigga's guidance that a wise king does not seek out war, and thus decided to call off the wars to instead live life as a wise and just ruler, much to the indignant rage of Cul, who saw his brother as becoming weak and soft-hearted, while Hela angrily argued that she needed to restore her lost cousin Loki's honor by carrying out her king's orders, but both were denied their case.
A few days later, Odin would seek out the Fates at Mimir's Well to know what was to happen in the future and if his sons/the other Asgardians would ever be revived; the three ancient hags told him that fate had decreed they be reborn not as full adults, but as children so as to live their lives anew as new versions of their previous selves, and they would need Odin's guidance if they were to perform their quests to the best of their ability. However, the Fates also prophesied that the first of the Aesir to be reborn as a child would be Balder, and if he were to be killed by one who was very close to Odin, then Ragnarok would begin as the dread Fire Giant Surtr would take that as an opportunity to light his Twilight Sword upon the Eternal Flame and reignite the war to end all wars so soon after the worlds were just starting to rebuild themselves from the earlier conflict.
Desperate to save his family and the universe by any means, Odin waited until Balder was reborn as a babe and have him sent to Midgard to live as a hermit, keeping him safe for millennia away from his relatives' grasp. Now Hela by this point was already starting to crave power for herself, and had heard the prophecy of Ragnarok from the whispers of a very familiar shadow lurking in the catacombs beneath the castle, so when she found out what Odin had done to avert fate, she lost her grip on her sanity and attacked her uncle, intending to take the throne now more than ever. Instead, she was defeated and forced to resume her mythological title as ruler of Niflheim, the Norse equivalent of the Underworld, without any means of escape, but before she left to perform her cursed duties, Hela vowed that once Odin died, she would not rest until Balder was hunted down and killed, even if she waited a millennia for his continued survival.
In the several thousand years since, Thor and Loki were also reborn as children and grew up unaware of their lost sibling, all while Balder lived in the wilderness of Scandinavia as a hermit, unknown to the world but living eternally and beautiful as his original self and equally unaware of his lineage or his destiny.