Lynn Reinweiß is the only daughter of the noble Reinweiß family and the main heroine of The Case Book of Arne. She spent her childhood as a carefree and innocent girl, but after losing her mother two years before the events of the story, she worked hard to present herself as a proper and refined daughter of a prestigious house. She lived a sheltered and lonely life in her family's mansion until a fateful night when she noticed her father acting strangely and decided to follow him, only to witness a grisly murder. Pursued by a mysterious figure, she fled into the supernatural city of Lügenberg, where spirits and monsters dwell. There, she encountered the vampire detective Arne Neuntöte and formed an alliance with him to solve the murder and uncover the truth behind her family's secrets.

Lynn has a bright and affectionate personality. She is characterized by her bravery and willingness to step into the dangerous supernatural world despite being an ordinary human. She possesses a keen intuition that allows her to quickly notice people's secrets and hidden emotions, and she provides the emotional insight and human perspective necessary to navigate social nuances that Arne, with his cold and analytical vampire logic, often overlooks. She also harbors an extreme fascination with vampires, which can cause her to become exuberant and leave her companions bewildered. This enthusiasm does not detract from her seriousness when facing danger, and she consistently demonstrates courage in the face of threats.

Her primary motivation is to seek the truth about her family's secrets, particularly the actions of her father, which set the central mystery in motion. As the story progresses, she becomes deeply involved in solving supernatural cases alongside Arne, driven by a desire to understand the world she has stumbled into and to find her own place within it.

Lynn serves as the central heroine and often acts as the viewpoint character for the audience. She is Arne Neuntöte's indispensable assistant, acting as a bridge between his detached supernatural reasoning and the human society in which they operate. Her role is crucial for navigating cases that require empathy and social understanding, complementing Arne's deductive prowess. She also forms key bonds with other characters. She has a competitive and teasing relationship with Nacht, Arne's small familiar creature. She shares a grounded and emotionally supportive bond with Louis Hartmann, another human who becomes involved in the investigations, often serving as his guide through the strange world of Lügenberg. Her connection with Arne is built on a contract that binds their fates together through the solving of mysteries, and she gradually challenges his detached perspective, helping him adapt to having a human companion in his work. She is also a target of the antagonist Zisye, who uses her to provoke Arne.

Over the course of the series, Lynn undergoes significant development. She begins as a vulnerable girl trapped by her noble lineage and sheltered upbringing, seeking help in a moment of crisis. As she becomes more involved in investigations, she evolves into a capable and autonomous investigator who can hold her own in the supernatural realm. Her journey is defined by her growing confidence, her search for the truth about her family, and her increasing ability to navigate the blurred boundaries between the human and non-human worlds.

Lynn does not possess supernatural powers or physical combat abilities. Her strengths lie in her keen intuition, her emotional intelligence, and her bravery. She is able to notice details and emotional cues that others miss, and she uses her human perspective to solve mysteries that require an understanding of human nature. Her greatest asset is her ability to act as a bridge between Arne's logical vampire reasoning and the emotional, social complexities of the human world. She is also resourceful and resilient, willing to face the supernatural dangers of Lügenberg despite having no inherent defenses against them.