Berta Kästner, known as 009-4, is one of the prominent cyborg agents shown in the 009-1 series. She is part of the Nine Number Group, and exclusive to the anime adaptation. She is modeled after Albert Heinrich/004 from Cyborg 009.
Berta is a specialized combat cyborg and "human weapon", with built-in firearms and ammunition stored throughout her body. Her backstory was briefly detailed in the extra booklet that came with the sixth DVD release in Japan.
Berta was a child of poor immigrants, but rose to greatness in becoming a highly-active athlete. However, she was soon seriously injured in a traffic accident. The Zero Zero Organization offered to reconstruct her limbs with cybernetic surgery, with the condition that she join them as one of their intelligence agents.
- Her surname may derive from the German author Erich Kästner, although the surname has appeared with other German historical figures as well. Her given name Berta, while a form of "Bertha" ("bright"), can also be a diminutive of the name Alberta.