Douglas "Doug" Billingham is a seasoned investigator in Lisvalletta's Seven-O Special Crime Investigation Unit. Operating under the "Double Decker System," he partners with rookie detective Kirill Vrubel to combat the lethal drug Anthem. Known by the nickname "Veteran," Doug maintains a consistently laid-back, deadpan demeanor, often seeming aloof or disengaged during routine tasks. Despite this perceived laziness, he possesses exceptional deductive abilities, allowing him to swiftly analyze crime scenes and accurately predict criminal movements.

He relies on a modified .38 caliber handgun, transformed into Anthem-neutralizing bullets that reverse the drug's effects. Doug favors this weapon as the only tool "befitting a detective" and operates a gadget-equipped vehicle resembling a DMC DeLorean. Recurring traits include tapping his fingers against his head while thinking, frequent consumption of anpan (sweet bean paste bread), and drinking canned coffee.

Interpersonally, Doug engages in deadpan snarking and deliberate trolling, especially toward Kirill. Examples include fabricating a non-existent congratulatory party and misleading Kirill into guarding a robot colleague's secret long after others knew the truth, contributing to the narrator labeling him "kind of an asshole." However, his behavior shifts dramatically during investigations involving the criminal "Good-Looking Joe," where he abandons his usual detachment. This intensity stems from the loss of his previous partner to Joe, driving a personal vendetta that overrides his typical reluctance toward active engagement.

Throughout their partnership, Doug's dynamic with Kirill evolves from merely tolerating the rookie's enthusiasm to acknowledging his unexpected competence in genetics. This includes subtle indications of mutual respect during critical operations, though his deadpan humor and trolling persist. His backstory reveals no familial connections, focusing instead on his professional history and the formative trauma of his partner's death.