Galderbroug stands as the primary antagonist. He is affiliated with Merilot Life Insurance, an organization operating with its own agenda concerning the boundary between the human world and Grimwald, the realm of the Death Gods. His core motivation stems from a belief that extending human lifespans through conventional Grim Reaper activities ultimately leads to greater self-destruction among humanity. To prevent this outcome, he aims to dismantle the barrier separating the two worlds, intending to establish direct control over the human population.

He wields the Demon Sword Tyrfing as his primary weapon. His strategy involves kidnapping Mina Okura to use her as bait, specifically targeting Ryosuke Kaga to lure him into a confrontation within Grimwald. Galderbroug seeks to acquire the complete power of the legendary sword Gram, linked to Ryosuke. This plan encounters complications when Ryosuke manifests Gram's full power during the ensuing conflict, leading to a climactic battle. Galderbroug ultimately discovers his scheme has yielded unforeseen and significantly more dangerous repercussions than anticipated.

The confrontation results in a "grand triumph" for the opposing forces but simultaneously triggers a "terrible consequence" directly stemming from Galderbroug's actions and the battle's resolution. The series concludes without depicting further developments concerning his plans or fate beyond this pivotal conflict.