Derflinger, an ancient spirit sword forged through elven magic 6,000 years ago by Sasha—the inaugural Gandálfr—serves as a sentient weapon capable of consciousness transfer to new vessels upon destruction, a rebirth requiring weeks to complete. It harnesses the power to absorb spells, converting magical energy into reserves for offense or defense, though stronger enchantments challenge its efficiency. Discovered as a corroded blade in a Tristain shop, the sword is acquired by Louise for Saito due to its affordability. Once awakened, Derflinger allies with Saito, providing tactical acumen during skirmishes and unveiling insights into arcane lore, historical secrets, and hidden identities—from recognizing Void mages like Louise and elves like Tiffania to unmasking Sylphid as a Rhyme Dragon.

In pivotal battles against adversaries such as Fouquet’s golem and the Ancient Dragon, Derflinger proves indispensable, seizing control of Saito’s body via stored magic during unconsciousness to avert disaster. Shattered by the Ancient Dragon’s assault, its consciousness later resurfaces within a fighter jet’s control system, demonstrating adaptability to mechanized interfaces tied to Saito’s Gandálfr runes.

Tied to foundational Halkeginian history, Derflinger’s past intertwines with Brimir’s demise at Sasha’s hand, cementing its bond with the Gandálfr lineage and Saito as a destined wielder. The sword’s banter often nudges Saito and Louise about their unresolved romance, though it professes no firsthand understanding of love.

When assassins obliterate its form in later events, Derflinger’s essence persists within one of Saito’s katanas, reaffirming its identity as an eternal spirit guided by purpose over physicality. Its dialogue weaves practical battle strategies with enigmatic musings on fate, blending ancient wisdom with occasional lapses in memory—a testament to its millennia-spanning existence.