Haruto Saionji hails from the human realm of Elde, where he is the wealthy heir and Student Council President at St. Luminous College. He displays unmatched prowess in fencing and kendo, remaining undefeated against fellow council members. Despite their differing grades, he shares a notably close and open bond with underclassman Reia Hiruda.

He and Hiruda are transported early to End Earth via a magical book she discovered. Renaming himself "Traihard," he ascends to Emperor of Baelgard—a technologically advanced secessionist elven kingdom. His rise is fueled by his identity as a "Soul Blader," manifesting swords from the hearts of emotionally bonded individuals. He specifically wields blades drawn from Baelgard’s four guardians, including Hiruda.

As emperor, he launches a conquest of End Earth to save it from darkness, believing Soul Bladers like himself and classmates Souma Akizuki and Kaito Kiriya are destined to enforce a new order. He deploys Baelgard’s mass-destruction weaponry against rival nations, positioning himself as an antagonist. He even defeats Kiriya in a conventional sword duel.

His trajectory shifts when Killrain’s release of dark energy disrupts his conquest. Facing mortal peril while linked to a dark tower, he is rescued by Hiruda, who sheds her corrupted "Hildareia" persona. This act compels him to acknowledge and reciprocate her genuine feelings, forging a mutual connection that unlocks his ultimate weapon: the "Holy Grail," drawn directly from her heart.

He renounces his imperial ambitions, choosing to remain in End Earth alongside Hiruda, the artificial elf Xecty, and warrior Jin-Crow. Together, they dedicate themselves to restoring Baelgard’s forest, transitioning Haruto from conqueror to guardian.