Victor Steinberg is Mary Steinberg's older brother. Following a military accident, his auxiliary brain seized control of his body, becoming a distinct entity called Victor Steinberg (Shadow). Victor's original consciousness survives remotely, operating a hand-shaped extend named "Lefty," while the fate of his physical human body remains unknown; both consciousnesses rely on remote proxies.

In their youth, Victor was Mary's role model, showcasing exceptional mechanical skill and fervent belief in extend technology, inspiring her career as an extend engineer. During the war, he pioneered remote extend piloting technology to save lives, becoming its first test subject. An auxiliary brain, modeled after human hemispheres, was created to manage cognitive load but lacked safeguards for mental trauma.

Witnessing his remote-piloted extends commit mass murder devastated Victor, overwhelming his biological brain. The auxiliary brain absorbed this trauma, developing its own consciousness—the Shadow. This entity embodies Victor's scientific curiosity warped by the horrors of war into a profound hatred for all extends. Believing extends are a mistake, the Shadow collaborates with the anti-extend terrorist group Spitzbergen, orchestrating the destruction of numerous extends.

The Shadow's appearance is unsettling: exposed gums and teeth, leathery stapled skin, a single eye, and a mostly bandaged body revealing sickly green flesh. It retains Victor's engineering genius and wartime expertise, primarily using sickle-like limbs to disassemble extends from range or through contact, effortlessly reducing them to non-functional components.

Before the Shadow emerged, Victor served as a special engineer for Juzo Inui during Juzo's time as a Gun Slave Unit. He secretly commissioned Juzo to protect Mary, a fact unknown to her. The Shadow later re-enters Juzo's life as an antagonist under Spitzbergen's directives while being hunted by Berühren Corporation forces.