Nirvash begins as a small, two-legged Larva companion to the young Renton Thurston, capable of vocalizing "Mukyuu." Renton uniquely understands and communicates with it, establishing their deep bond. The Larva exhibits autonomous behavior, notably crashing into a building during a military mission, which directly causes Renton's reunion with Eureka after eight years apart.

Driven by Renton's intense determination to protect Eureka, Nirvash evolves into a more advanced, humanoid mecha form known as the Spec4. This form incorporates enhanced weaponry, including laser batteries similar to those of the rival mecha, TheEND. This evolution enables the Fusion Phenomenon, a critical event destroying numerous EIZO aliens in combat. Nirvash later battles Holland Novak piloting the Devilfish mecha while Renton is severely injured, demonstrating its responsiveness to Renton's resolve over his physical condition.

In the climax, Nirvash reverts to its larval state to confront TheEND, also in a diminutive form; both vocalize "Mukyuu" during their struggle. The Spec4 plays a pivotal role during the Gainax Ending sequence, contributing to reality's surreal reconfiguration, though Nirvash's ultimate fate remains undefined within the abstract conclusion.

Throughout, Nirvash acts as both physical protector and emotional catalyst for Renton and Eureka. Its dual nature—shifting between vulnerable larval companion and formidable combat mecha—embodies themes of innocence and sacrifice. Its origins as a creation linked to the EIZO ("Image") remain unexplored, with the narrative focusing on its bond with Renton and its instrumental role in the central conflict.