Jun Zaizen is the young heir to the global Zaizen conglomerate, also known as the Zaizen Group or Zaizen Konzern. A gifted and highly intelligent engineer despite his youth, he created the bioroids Megadeth and Troidal. These creations later become central figures in the criminal organization Id, resulting in a complex dynamic where Jun and the bioroids are considered brothers in a sense.

His life is irrevocably altered during a party held in his honor when he is targeted for assassination by Id's Crime Mobiles. The attack claims the life of his father, the chairman of the Zaizen Group, and leaves Jun mortally wounded. Three men—Ken Kanzaki, Ryo Aguri, and Go Goda—are also gravely injured while protecting him, an event that directly leads to their transformation into the cyborg unit Metal Jack.

Jun remains in intensive care for some time following the attack. A child prodigy, his sheltered upbringing left him with feelings of loneliness and social awkwardness. He develops a sense of affection for Ken Kanzaki's foster sister, Sayuri. His chief secretary, Shunichi Kiritani, holds a certain adoration for him and consistently addresses him as "Mr. Jun."

A significant turning point occurs when he is brainwashed in Episode 28 and becomes the leader of a new iteration of the terrorist organization, Neo Id. He also created a female android named Crola. Although she was later reprogrammed to serve Troidal in Neo Id, she was originally one of Jun's creations.