Saito Hiraga, an ordinary high school student from Tokyo, Japan, possessed unremarkable academic and athletic abilities. His life transformed when a mysterious portal appeared before him as he walked home carrying his broken laptop. Driven by curiosity, he touched the portal and was swept away to Halkeginia, arriving at the Tristain Academy of Magic. There, Louise de la Vallière forcibly bound him as her familiar through a ceremonial kiss, inscribing magical runes on his left hand.

Initially disoriented and resistant to his new role, Saito struggled to adapt to Halkeginia's feudal, magic-based society. Early defiance led to conflicts, including a duel with the noble Guiche de Gramont where his Gandálfr runes first activated. These runes grant instant mastery of any weapon, significantly enhancing his strength, speed, and durability during combat. This augmentation ceases immediately after use, often leaving him exhausted or unconscious if injured. The runes impose an instinctive subservience to Louise, though he frequently challenges her authority due to his disdain for Halkeginia's class system.

Saito wields Derflinger, a sentient sword acquired early on. Derflinger reveals Saito is the legendary Gandálfr ("God's Left Hand"), one of four familiars destined to serve Void mages, cementing his purpose as Louise's protector. After dying and being revived by an elf, Saito temporarily lost his runes and abilities. He regained them when Louise re-summoned him, but subsequently became bound to a second Void mage, Tiffania Westwood, acquiring the additional title of Lífþrasir ("God's Heart"). This dual bond allows him to serve both, though his primary allegiance remains with Louise.

Saito evolved from a resentful familiar to a recognized hero. After single-handedly thwarting Albion's invasion of Tristain, he was knighted as "Chevalier de Hiraga" and later granted the title "des Ornières" upon marrying Louise. His relationship with Louise, central to his development, is marked by mutual but often unspoken affection. Despite conflicts arising from his perverse tendencies and her violent outbursts, he demonstrates fierce loyalty, declaring his love and prioritizing her safety. His jealousy surfaces when other men show interest in Louise, and he actively prevents her arranged marriage.

In *Rondo of Princesses*, Saito's knight status creates tension with Louise as they redefine their relationship. His Gandálfr runes begin fading, prompting a quest for restoration. This season introduces heightened romantic rivalries with figures like Queen Henrietta and Tiffania. Saito's adaptability is tested confronting another Void mage, Sheffield, and navigating political intrigue. His resolve to protect Louise remains unwavering despite threats to their bond.

Saito's personality blends curiosity, impulsiveness, and idealism. He rejects Halkeginia's nobility-based hierarchy, defending commoners like the maid Siesta. Though he exhibits perverse tendencies, such as fixating on female physiques, these stem from his teenage perspective. His resilience is evident in withstanding Louise's abuse, rescuing allies against impossible odds, and overcoming isolation during brief returns to Earth, where he realizes a world without Louise holds no meaning.