Derflinger, commonly known as Derf, presents as an inexpensive sentient sword purchased by Louise for Saito. His true nature is revealed as an ancient magical artifact of significant historical importance, having been the partner to the previous Gandálfr familiar 6,000 years earlier. This establishes a destined connection to Saito, the current Gandálfr.

Derflinger absorbs and nullifies magical spells targeting his wielder, a vital defensive power repeatedly employed during Saito's battles and crucial for protecting Void mages like Louise during their spellcasting. He speaks with a casual, often humorous demeanor and straightforward style, frequently offering tactical combat advice while commenting on Saito's relationships and situations with a mix of wisdom and sarcasm.

Acting as a mentor, Derflinger provides Saito insights into the functions of the Gandálfr runes and the history of Brimir's Void magic, actively guiding him in mastering his familiar abilities. Shattered during the confrontation with the Ancient Dragon, Derflinger later reveals his consciousness survived by retreating into the Gandálfr runes on Saito's hand. He eventually re-manifests in a new physical form, continuing their partnership.

His existence remains intrinsically linked to Saito beyond their physical bond, enduring through the series' conclusion where Saito wields him in the final battle against the Ancient Dragon using an F-2A Jet Fighter.