Berta Kästner, codenamed 009-4, operates as a cyborg agent for the Western Bloc within the all-female intelligence unit, the Number Nine Group, during a prolonged Cold War. Her unit colleagues include Mylene Hoffman (009-1), Vanessa Ibert (009-3), and Mia Connery (009-7).

She possesses extensive cybernetic modifications, with all four limbs enhanced to incorporate tactical units at her elbows and knees; these mission-specific modules provide combat and espionage adaptability. Among her unit, she has the highest number of cybernetic enhancement points. Her design motif is inspired by Albert Heinrich (004) of the unrelated "Cyborg 009" series, emphasizing comprehensive body weaponization.

Her documented missions target the rival Eastern Bloc. One early assignment involved rescuing Zond Soyuz, a Russian defector and scientist who developed a potent energy source coveted by the Western Bloc. Soyuz was held captive by Eastern Bloc operative Ivan Godunov. Berta Kästner joined an infiltration team that raided Godunov's castle stronghold. The mission involved fierce combat against Godunov's forces and Lyudmila Schindler, the Eastern Bloc's Director of Intelligence. While the team failed to extract Soyuz alive, they secured his invention and thwarted a plot to disrupt a lunar disarmament summit.

Physical descriptions note her noticeably darker skin tone compared to other characters. There is no documented background, origins, pre-service history, or significant character development arcs across official media. Details about her personality, motivations, relationships beyond operational partnerships, or evolution across appearances are also absent.