Acheron

Acheron is an exact duplicate of Pluto created by Shadow. Unlike Pluto, Acheron was built without Kokoro programming. He is much more dangerous and very hard to control. He appears exclusively in the 2003 Astro Boy anime series, filling the role normally filled by Bora in the Greatest Robot in the World arc.

In the original Japanese dub, Acheron is named "Dark Pluto". In the international versions, Acheron is named after a river in Greece; this river is famous in ancient Greek mythology and is said to be one of five rivers to the underworld.

Personality
Acheron behaves like a much colder Pluto. While he has the same voice and base characteristics, he has no developed emotions, due to his lack of kokoro programming. He simply follows basic orders that have been pre-programmed into his system

Appearance
Acheron is almost completely identical to Pluto. However, his colour scheme is changed to be mostly made up of dark blue and black. His eyes are plain lights, as opposed to Pluto's expressive eyes with pupils.

Series
Acheron appears exclusively in the episode, "The Fall of Acheron". In this episode, Shadow ruminates on how Pluto spared Astro's life in a previous episode. Shadow had predicted that Pluto's kokoro programming would become his weakness, so Shadow made a backup version of him. Shadow kept this copy in stasis until after Pluto spared Astro's life at the end of a fight.

After this, Shadow sends out Acheron to replace Pluto in his battling journey. Pluto intervenes, sacrificing himself in order to save Astro and to take down Acheron, seemingly destroying both original and duplicate. Shadow later comments, "Kokoro isn't a weakness, it makes robots stronger."

Trivia

 * Archeron's overall colors invoke that of Pluto's designs in previous iterations of Astro Boy, while Pluto himself was recolored green for the 2003 anime series.