Lügner is a demon who appears in the early part of the story, serving under the powerful demon known as Aura the Guillotine, one of the Seven Sages of Destruction. His name comes from the German word for liar, and this meaning directly reflects his role and nature. He is a blond-haired demon who initially presents himself as a calm, polite, and reasonable peace envoy sent to negotiate with Graf Granat, a human noble whose territory has been in conflict with Aura's forces. This courteous demeanor, however, is a calculated act. As a demon, Lügner is incapable of experiencing genuine human emotions such as empathy, guilt, or compassion, yet he has become highly skilled at mimicking them in order to deceive humans and achieve his objectives.

Lügner's personality is defined by a cold, analytical rationality and a deep sense of pride in his magical abilities. He views humans primarily as tools or obstacles to be manipulated or eliminated. He is observant and strategic, carefully assessing his opponents' strengths and weaknesses before committing to action. His calm exterior hides a ruthless and arrogant nature, as he believes that his centuries of magical study place him far above any human practitioner. This pride is a central aspect of his character and ultimately contributes to his downfall. He holds a particular disdain for those who achieve magical prowess through natural talent or shortcuts, valuing instead the long, disciplined path of study that demons traditionally follow.

His primary motivation is to reach the legendary Golden Land of Ende, a mythical location that even the former Demon King could not attain. This goal drives his actions and his alliance with Aura, whom he serves not out of loyalty but out of a pragmatic alignment of interests. He leads a small group of demons, including a young-looking demon named Linie, who works alongside him as a partner. Their relationship is not based on friendship but on shared purpose, with Linie acting as a scout and combatant while Lügner handles strategic planning and psychological manipulation.

In the story, Lügner first approaches Graf Granat under the pretense of seeking peace, but his true aim is to infiltrate the human stronghold and disable its magical barrier, clearing the way for Aura's forces. His role in the narrative is to serve as an early, significant threat to the main party, demonstrating the true danger that demons pose. He is the first opponent to truly test the younger members of the group, particularly the warrior Stark and the mage Fern.

Lügner's key relationship within the demon group is with Linie, a demon who can transform into a monstrous form and copy fighting techniques. Together, they work as a coordinated unit during combat. On the human side, his primary adversaries are Fern and Stark. He engages in a psychological battle with Stark, attempting to exploit the young warrior's deep-seated insecurities and fear of inadequacy. This confrontation forces Stark to confront his own cowardice and ultimately helps him grow into a more confident fighter. Lügner's most important direct confrontation, however, is with Fern. Initially dismissing her as weak based on her suppressed magical energy, he is caught off guard when she reveals her true speed and power in casting offensive magic, a technique taught to her by Frieren.

Lügner's notable ability is a blood-manipulating magic, which allows him to shape his own blood into weapons such as blades or whips, form shields for defense, and repair his own injuries. He can also mark opponents with his blood to track their movements. This magic is versatile and efficient, reflecting his methodical and resourceful nature. He is a highly disciplined mage who has spent a long lifespan perfecting this single art.

Over the course of his appearance, Lügner shows little to no emotional development. He remains steadfast in his demonic nature and his contempt for human methods until the very end. His defeat comes when Fern overwhelms him with a relentless barrage of magic during a moment of distraction, after Linie is killed by Stark. In his final moments, he realizes that Fern and Frieren had been concealing their true magical power, a tactic he considers dishonorable. His dying words express anger at what he perceives as deception, an ironic fate for a character built entirely on lies. Lügner's primary significance in the story lies in how he embodies the core themes of the series regarding demons, serves as a catalyst for the growth of Stark and Fern, and provides an early demonstration of the deceptive nature of magic in the world.