Sepia is a warrior of the all-female Cthulhu species (also known as Cutowolf or Cthuwulf), serving their leader Sir Violet. She shares a profound emotional and intimate bond with her lover, fellow Cthulhu warrior Cobalt, established before the story begins.

This bond is shattered when Cobalt perishes during the failed invasion of Earth; her mecha, Delos Theta, is destroyed by Iczer-1 and her partner Nagisa. Grieving Cobalt's loss, Sepia is assigned by Sir Violet to command the Cthulhu fortress Nova and is partnered with Iczer-2, an artificial being created to oppose Iczer-1. Together, they synchronize within the giant robot Iczer-Sigma to battle Iczer-1 and Nagisa piloting Iczer-Robo.

During a critical confrontation, Iczer-2 callously crushes Sayoko, a young human girl Nagisa was protecting. Witnessing this cruelty breaks Sepia's resolve and morale. When Iczer-Sigma suffers critical damage, Sepia refuses teleportation to safety. Choosing to remain aboard the doomed mecha, she seeks reunion with Cobalt in death. Sepia perishes in Iczer-Sigma's explosion. Her story centers on her relationship with Cobalt, her grief-driven role alongside Iczer-2, and her ultimate refusal to survive, leading to deliberate self-sacrifice.