Sissel Grauer is the younger sister of Innumael Grauer, a smuggler within the transportation sector. She lives at her parents' home during her brother's absence. Innumael maintains a close, protective, and clearly affectionate bond with her despite his criminal life.

Captain Helman of the Wahrheit Imperial Army's Zeroth Platoon abducts Sissel, using her as a hostage to force her brother's surrender and execution, leveraging her familial importance against Innumael.

This abduction compels Leocadio, an imperial soldier initially involved in Innumael's arrest, to intervene. Faced with Helman's extreme methods, Leocadio kills his commanding officer to free Sissel, an act that renders him a fugitive. Following her rescue, Sissel's subsequent fate or further role is not documented.

Physically, she possesses black hair and olive-colored eyes. Her character design underscores her youth and vulnerability within the story's conflict.