Hel emerged as an alter ego within Nagisa Natsunagi's body, created from Nagisa following Alicia's death roughly six years before the main events. She ranks among the highest officers in the secret organization SPES and is designated a potential vessel for SEED, a core entity to SPES's goals.

Physically, Hel has short black hair and distinctive red eyes that visibly glow during combat or ability use. She wears a wine-red military uniform with a service cap bearing a golden emblem, multiple sabers typically secured at her waist, long black leggings, and boots, presenting a sharp contrast to Nagisa's appearance.

Beyond enhanced strength and combat skill, Hel wields a form of mind control, enabling her to dominate others' actions against their will. This power has limitations, failing against Assassin Fūbi Kase due to strong resistance. Hel's physiology also permits coexistence with other identities in the same body, creating complex internal dynamics.

Hel's background shows a direct link to Alicia, existing as Alicia's alternate personality before Alicia died. Hel's consciousness then transitioned into Nagisa Natsunagi's body, establishing a tense dynamic where Hel and Nagisa share one form. Nagisa represents a new identity that emerged to replace Hel, leading to conflict over control.

Functioning as a primary antagonist and leader within SPES, Hel engages in direct confrontations with her archenemy, Siesta. A pivotal conflict saw Siesta sacrifice her life to suppress Hel's influence and protect Alicia's legacy. This act temporarily restrained Hel's dominance, though her presence persisted within Nagisa. Subsequent developments show Siesta, through mechanisms linked to her sacrifice, gaining the capacity to intermittently override Hel's consciousness, further complicating the internal struggle for control.

Her name derives from Norse mythology, referencing Hel, the female entity ruling the underworld realm of the same name, aligning with her role as a harbinger of demise.