Sleipnir was born in Asgard to Loki (mother, in the form of a horse) and Svaðilfari (horse) after Loki attempted to stop a builder from constructing a wall in Asgard. He is Loki's first child. Sleipnir grew up happy on Asgard until Odin wiped Asgard’s memories and separated Loki’s children. Ever since then, Sleipnir remained in the royal stables, serving as Odin’s loyal steed until Odin died, thus breaking the spell.

Freed from the curse, Sleipnir shapeshifted to an Asgardian form and traveled to Alfheim to find Jormungandr. Together, they wound up in Sakaar, where they were forced to battle. Though he had no prior experience, Sleipnir quickly learned how to hold his own alongside his brother. Initially at odds, Sleipnir’s relationship with his brother grew stronger after Jormungandr accidentally bit him when trying to bite the Hulk. Sleipnir survived the venom, and they escaped to Asgard by manipulating a portal. There, they found Asgard in ruins with their half-sister, Hela, attacking Surtur. Sleipnir managed to bring them all, including Fenris and Hela, to the Afterlife, where they resided until after the threat of Thanos passed.

Always a passionate scholar, Sigyn loved to learn. He strives to see others' points of view before making assumptions. He is trusting and caring, often taking care of his younger siblings. He is a natural-born leader, even though he prefers to let others occasionally take the reins. Sleipnir has unending patience, and it takes a lot to upset him. However, if anyone messes with his siblings or hurts his family and friends, he is more than capable of defending himself. His trusting attitude sometimes gets him in trouble, but he still tries to give others the benefit of the doubt. With the help of his younger siblings, he has toughened up while still keeping his patience and kindness.

Even though he prefers to compromise before fights, Sleipnir is skilled in hand-to-hand combat, swordsmanship, and the ability to create and manipulate portals/rifts in space. Sleipnir boasts the strength, speed, and agility to hold his own in a battle. He had Asgardian longevity and is capable of moving at quick speeds on water, air, and land. If provoked, Sleipnir has proven himself to be a worthy warrior.