Kili Surtr Muspelheim is a central figure in the Unlimited Fafnir series, initially standing as a primary antagonist before her role evolves significantly. She is recognized as the most dangerous among the Ds, a group of girls who possess the powers of dragons, and is specifically labeled as a disaster. As the leader of the Sons of Muspell, a cult that worships dragons, she orchestrates operations to further the group's enigmatic goals, which often put her in direct opposition to the main characters at Midgard, the academy for Ds.

Kili possesses a formidable and versatile set of abilities derived from her nature as a D. Her primary power involves the use of dark matter to perform biogenic transmutation. This ability is incredibly potent, allowing her to manipulate matter on a biological level for a variety of purposes. She can create flames for propulsion, enabling her to fly, and more critically, she can heal her own wounds and replace lost blood. However, this power is not without limits; it requires intense concentration and continuous use places a significant strain on her mind, leading to exhaustion when pushed too far.

Personality-wise, Kili is a complex individual. While she leads a dangerous cult and is regarded with significant caution by authorities like NIFL, her actions are not driven by simple malice. She demonstrates a self-aware and often self-deprecating sense of humor, occasionally mocking her own circumstances and choices. Despite her reputation, she places a high value on the bonds she forms with others, perhaps because she feels her own life was "born out of nothingness" and has been hollow. This drive leads her to cherish connections, which explains why she goes out of her way to rescue someone like Jeanne Hortensia, even when she has no obligation to do so.

Her motivations are deeply tied to these connections, particularly with the protagonist Yuu Mononobe. Three years before the main story, she met Yuu, an encounter that left a lasting impression on her. When she is severely injured and exhausted from a battle, her immediate thought is to seek him out, considering him and Midgard to be the most trustworthy party, even though she openly admits she probably should not trust them. This internal conflict—knowing she should not trust her enemies but feeling drawn to them anyway—is a central part of her character. Her relationship with Yuu progresses from adversarial to cooperative, and she eventually enrolls at Midgard, being marked as one of his mates and becoming a member of the Brynhildr Class. This transition marks a significant development from her origins as the leader of a dragon-worshipping cult. Her connection to the dragons themselves is also notable; she serves as a relay for controlling the monster Hekatonkheir, and the dragon Vritra even uses her face as a reference for its own form.