Sheffield acts as Void Familiar to King Joseph of Gallia, hailing from a technologically advanced human society beyond the Sahara Desert, though one still surpassed by elven capabilities. Her unique power, Myozunitonirun (Mind of God), enables her to operate any magical artifact and accomplish nearly any magical feat.

Initially posing as Lord Cromwell's secretary during the Reconquista uprising in Albion, she secretly masterminds the movement as its true leader. After Cromwell's imprisonment, she executes him, seizes control, and retrieves the Water Spirit's ring from his body. Her unwavering loyalty to Joseph is driven by a profound, yet unfulfilled, desire for his love.

Sheffield stands as the sole known character capable of merging Void magic with Firstborn magic, creating entities like the Jormungandr golems and Firestones. When detonated using Void incantations, these Firestones unleash catastrophic destructive power, capable of annihilating entire airship fleets. Her runes become visibly apparent on her forehead during the use of her abilities.

Following Joseph's defeat by Saito, she detonates a Firestone, killing both Joseph and herself in an act of mutual destruction. Throughout her involvement in Gallia's political schemes and her manipulations of events in Albion and Gallia to further Joseph's ambitions, her actions consistently align with his goals, anchored by her unreciprocated emotional attachment.