Rumis functions as an animal god overseeing Crystania, having adopted a serpentine form after ancient divine conflicts compelled defeated deities to shed their original bodies and bind their souls to the land's creatures. His core responsibility involves shielding Crystania from external dangers, culminating in the creation of the Gods' Wall—a colossal barrier mimicking snake scales that isolates Crystania from neighboring realms like Da'nan. This structure physically enacts his mandate to bar outsiders from the sacred land.

Enforcing Rumis's will are his followers: lizardmen and humans adept at shifting into reptilian shapes. They relentlessly hunt intruders breaching the Gods' Wall, exemplified by assaults on Redon's group upon their arrival in Crystania. Rio, a human commander directing these forces, orchestrates attacks under Rumis's guidance. Despite this authority, Rumis displays wariness when sensing the dominant god Barbas's presence near Redon's party, retreating immediately—revealing his subservient status in Crystania's divine order.

Rumis's failure to stop Redon's group from crossing the barrier—forced open by Barbas—and his inability to eliminate them expose his limitations. His enforcers, including Rio, persistently target the intruders throughout the film, but their repeated defeats and Rio's eventual death underscore the crumbling of his protective control against Barbas's overarching designs.