Sigil hails from an unnamed doomed planet like Hikaru, but was taken by the space dictator Friedshalf instead of human parents. Freed by him from a slave religion, this forged her deep debt and loyalty, shaping her into a formidable antagonist initially tasked with retrieving the Royal Armor protected by Hikaru.

She strategically infiltrates Hikaru's high school as a transfer student to locate the armor. Direct interactions with Hikaru expose Sigil to kindness and familial bonds absent under Friedshalf. A pivotal bath-sharing moment leads to conversations about their shared origins and vulnerabilities. Witnessing Hikaru's genuine care and experiencing a Gran Knight-induced vision revealing Friedshalf's destruction of Hikaru's royal family trigger internal conflict over her loyalty.

This conflict culminates in a decisive Heel Face Turn during Friedshalf's Earth attack. Confronting his destructive ambitions and failing to sway him, Sigil chooses to protect Hikaru in the final battle. She solidifies this shift by intercepting Friedshalf's fatal attack aimed at Hikaru, sacrificing herself. Her ultimate fate remains ambiguous after the stabbing, alongside Friedshalf's unresolved demise.

Sigil's complex relationship with Friedshalf blends Subordinate Excuse with hinted romantic feelings, making her betrayal emotionally charged. Conversely, her bond with Hikaru evolves from adversarial to deeply empathetic, symbolized by physical gestures like Marshmallow Hell embraces and shared vulnerability. Her internal struggle between duty and newfound moral conviction defines her character arc.