Klaus emerged from orphaned poverty in the slums, recruited and remade by the spy Guido. Guido forged him a new identity and subjected him to intensive training, ultimately embedding him within the elite spy unit Inferno. Inferno became Klaus's first true family, though he never bested Guido in combat during their association. He alone survived the unit's destruction on an "Impossible Mission," a result of his separate assignment during their final operation.

His physique is tall and lanky, marked by an androgynous, often feminine presentation: fair skin and shoulder-length black hair that habitually obscures his left eye, frequently causing misidentification. He reliably wears practical attire: a white collared shirt, black vest, black tie, and black shoes.

Near-constant calm and stoicism define Klaus's personality, with overt emotions rarely surfacing. Despite his reputation as the world's greatest spy under the codename "Bonfire," he finds teaching profoundly difficult due to his intuitive mastery of espionage, which he struggles to deconstruct or explain. He possesses sharp observational skills yet exhibits significant social obliviousness, exemplified when he interpreted a student's invitation for "a good time" as an offer to play chess. He experiences no romantic or sexual attraction and actively sets boundaries to prevent such entanglements with students. Familial bonds forged through shared hardship are paramount to him; he transferred this loyalty from the lost Inferno to his new team, Lamplight, whom he considers family. He deeply cares for their well-being and potential while maintaining professional distance.

His skills substantiate his legend: exceptional physical prowess, strategic foresight, analytical deduction, and combat proficiency. Documented feats include eliminating ten assailants with a crowbar within sixty seconds and emerging unscathed from an ambush by seventy-three operatives. His only acknowledged superior in direct combat was Guido, whom he later surpassed after an injury slowed Guido's reflexes. He also engages in painting, particularly during periods of grief following Inferno's loss.

Relationships are central to his journey. His bond with Guido blended mentorship, paternal reliance, and eventual betrayal. Within Inferno, he held deep respect for the leader Hearth, viewing her as a fundamentally kind yet stubborn mother figure, and rejects others using her title "Boss." His interactions within Lamplight vary: he appointed Lily leader due to her influence and employs her as a strategic wildcard; he respects Annette's ruthless practicality as a necessary "weapon of last resort" despite ethical concerns; he values Sara's animal handling skills and shares her affection for animals; he supports Erna's misfortune-sensing abilities, offering her protection; he deflects Grete's persistent romantic advances while genuinely finding her true face beautiful and caring for her deeply as family; he remains impervious to Thea's seduction attempts but trusts her empathy and field command potential; and he views Sybilla as kind-hearted but impulsive. Externally, he shares mutual animosity with the soldier Welter Barth, whom he considers a "pompous ass." He holds particular disdain for the assassin Roland, codenamed "Deepwater" (a deliberate foil to his "Bonfire"), dismissing him as an immoral "pushover" and rejecting his self-proclaimed rivalry.