Gagyson began existence as a minor Hebrew plague deity, later cast into demonic servitude as a subordinate within infernal hierarchies, notably serving Arioch per *The Book of Abramelin*. In the anime *Tokyo Revelation*, he operates as Ose's minion, orchestrating events to summon his master. He puppeteers the human Akito Kobayashi, using him to gather magnetite through sacrificial acts, such as unleashing a fatal plane crash.

Gagyson secretly works against Akito, conjuring demonic assassins to eliminate the perceived threat Kojiro Souma, fearing Masakado's awakening within him. This betrayal involves dispatching Cerberus to attack Kojiro, leading to Patra the dog's death—though Cerberus later binds itself to Patra's corpse to shield Kojiro. His sabotage extends to callously discarding Saki Yagami's possessed body after exploiting her as a parasitic demon conduit, severing Ose's magnetite flow and crippling the summoning ritual.

During Ose's battle against Kojiro (as Masakado) and Cerberus, Gagyson surrenders his life force to empower Ose, transforming him into a winged, heightened form. This sacrifice ultimately fails when Akito revives Saki as the goddess Konohana Sakuya, enabling Ose's defeat. Gagyson's manipulations thus directly precipitate his master's downfall.

Beyond the anime, Gagyson features in several *Shin Megami Tensei* games. *Devil Survivor 2*'s compendium echoes his origins as a plague god and subservience to demons like Orias or Ose, maintaining his low-rank status. His combat repertoire typically aligns with the "Fallen" race, wielding electrical assaults like Zio and Mazio alongside status-inflicting skills such as Pulinpa. Stat distributions in titles like *Shin Megami Tensei II* and *if...* emphasize moderate agility and magic, revealing weaknesses to force and curse elements.