Cobalt is a soldier of the all-female Cthulhu alien race, serving under the mind-controlled leader Sir Violet during Earth's invasion. Piloting the giant mecha Delos Theta, she leads military assaults after the initial parasitic infiltration strategy using Vedim creatures fails. Delos Theta's primary weapon generates explosive flashes causing massive destruction, devastating Tokyo and overwhelming human forces.

Cobalt shares a romantic and intimate bond with fellow Cthulhu soldier Sepia, depicted in scenes establishing their emotional connection. During battle against Iczer-1 and her synchronization partner Nagisa Kano, an energy beam attack fueled by Nagisa's rage over her parents' murder cripples Delos Theta. Nagisa then delivers the final blow, destroying the mecha and killing Cobalt.

Cobalt's death profoundly impacts Sepia, fueling her grief and later partnership with the burgundy-haired android Iczer-2. Sepia's desire for vengeance against Iczer-1 and Nagisa stems directly from this loss. Ultimately, Sepia chooses to perish alongside the destroyed Iczer-Sigma mecha to achieve spiritual reunion with Cobalt.

The prequel manga *Golden Warrior Iczer-One* by Rei Aran features Cobalt, originally depicting the character as male before the OVA reimagined the Cthulhu as all-female. Narrative details or development for Cobalt within the manga remain unspecified.