He is an associate of Elizabeth Xander.
Kestner worked for Poseidon's Halcon Laboratories, using cybernetics to rehabilitate traumatized victims of the last global war. However, Halcon's pride in their work eventually drove them to fanaticism, the division becoming obsessed with mind control. However, after the project lead Elizabeth Xander was killed in an accident, Poseidon decided to dissolve Halcon. Unfortunately, this did nothing to still Kestner's fanatical beliefs. He came to the conclusion that the cause of conflicts was individual difference and the only way to prevent future wars was to eliminate it entirely.
To that end, Kestner led his Halcon colleagues in secret, presumably designing the Connexus, a multimedia device that secretly contained a biochip that could overtake the mind and bodies of humans and cyborgs once activated. They also resurrected Xander as a cyborg to act as the control signal for when their grand plan was complete.
Halcon managed to hack digitally vulnerable cyborgs to carry out terrorists attacks around the world. One of these attacks left ESWAT member Briareos Hecatonchires seriously wounded. Kesnter conveniently operated on Briareos when he was brought back to Olympus for medical care. As Briareos' cybernetic systems were too advanced to be digitally hacked, Kestner secretly implanted nanites within his bloodstream, which would activate and take control of Briareos whenever his adrenaline picked up. Eventually, during a security conference at Tartaros, in which world leaders were gathering to discuss granting Olympus control of their satellite systems for counter-terrorism measures, Kestner used robotics doves to transmit the Halcon signal to Connexus users, even Manuel Aeacus, within the vicinity of the conference to order them to attack.
The attack successfully convinced world leaders to grant control of their satellites to Olympus. Kestner then infiltrates the satellites' backup server room disguised as a technician, having hacked the camera feeds to make him appear to be someone else. At the same time, a remote-controlled truck attacks Briareos, causing his nanites to activate and take control of him, forcing Briareos to crash the main server and force Olympus to switch to the backup.
After Briareos escaped the hospital and confronted Kestner at his apartment after figuring out his involvement in the attacks. Kestner confessed everything, shortly before committing suicide before the police arrive, making it seem like Briareos murdered him.