Loki Yozenheim is a high-ranking officer within the NIFL military organization, holding the rank of Major. He serves as the former superior officer of the protagonist Yuu Mononobe during Yuu's time as a soldier, and he later takes on the role of a major antagonist before evolving into a more complex figure who ultimately opposes Yuu as a self-styled hero. His background is largely tied to his work with the special forces unit Sleipnir, which he commanded and used to send Yuu on numerous missions across the globe over a span of three years, with the explicit goal of developing Yuu into the perfect Fafnir. Loki considers Yuu to be the best Fafnir, and he views results as the only thing that matters, often prioritizing mission success over any personal considerations. He is a pragmatic and calculating individual, focused solely on his duties and the objectives assigned to him by NIFL. His motivations are initially aligned with his organization's interests, but as the story progresses, he begins to act on his own agenda, particularly in relation to the Kraken Zwei threat and the fate of the Ds. He is known to be cold, direct, and willing to make ruthless decisions if they serve his purpose. Loki's primary abilities are not supernatural—he is not a D—but rather stem from his tactical acumen, leadership of the Sleipnir squad, and his influence within the military hierarchy. He commands a unit of silver-armored soldiers that resemble the Hreidmar type and often operates from the shadows, orchestrating operations and manipulating events from behind the scenes. His relationship with Yuu is central: he was once a mentor-like figure who shaped Yuu's killer instincts, but their dynamic shifts as Yuu grows beyond his control, leading to a direct conflict of beliefs when Loki later stands in Yuu's path. Other key relationships include his interactions with the researcher Miyazawa Kenya, with whom he exchanges veiled warnings about the dragon marks, and his connection to former Sleipnir member Jeanne Hortensia. Over the course of the series, Loki's role evolves from a seemingly straightforward antagonist into a more ambiguous and antagonistic force driven by his own interpretation of necessity and duty, culminating in a confrontation where his identity as a new "hero" is revealed.