Noël Kannagi is a fifteen-year-old corporal serving in the Helvetian Army's 1121st Platoon, where she works as the unit's mechanic and primary driver. From a young age she displayed a deep fascination with science and machinery, and her talents were quickly recognized at the Helvetian Academy. However, the military exploited her genius to restart the development of a biological weapon known as the Invisible Reaper, a project that resulted in thousands of deaths. Because of her involvement, Roman soldiers gave her the feared epithet the Helvetian Witch. This past weighs heavily on Noël, leaving her with a profound inner conflict and a deeply reserved demeanor.

She is naturally taciturn and often appears detached, spending most of her time in the fortress hangar repairing the platoon's multi-legged tank, the Takemikazuchi. Noël tends to work through the night and can often be found asleep during the day when not required for other duties. Despite her quietness, she answers when spoken to and occasionally offers dry humor. Her motivation stems from a sense of duty and a quiet desire to atone for her past by using her skills to restore and maintain the tank for defensive purposes rather than destruction.

In the story, Noël serves as the technical backbone of the platoon, keeping the Takemikazuchi functional and piloting it during combat. Her relationship with the new recruit Kanata Sorami is pivotal; Kanata's unwavering friendliness gradually helps Noël emerge from her shell and confront her guilt. Noël also shares a close, almost affectionate bond with the Takemikazuchi itself, treating the machine with the care one might give a living companion. Over the course of the series, she develops from a withdrawn, guilt-ridden figure into a more open member of the squad who finds purpose in protecting the peaceful life in Seize. Her notable abilities include a genius-level understanding of ancient technology, mechanical repair and improvisation, and skilled piloting of the platoon's walker tank.