Cobalt

Cobalt (a.k.a. jetto) was the brother of Astro Boy.

1960's series
Cobalt Jetto is Astro's older brother. He became overshadowed by Astro's brother relationship with Atlas in the 1980s Anime remake. There are two different stories to Cobalt's birth and purpose.

The first one, the Manga plotline, shows that after Astro disappeared, Professor Ochanomizu made a rough copy of him to deffuse a bomb. However, Cobalt's eletronic brain isn't as advanced as Astro's and he can't focus on detailed work, like defusing bombs. Professor Ochanomizu notes that "he doesn't have all of Astro's powers" but doesn't explain which of these Cobalt does and does not have. Because he and Astro are made from the same design, Cobalt is sent to track Astro and finds him in the ocean searching for the bomb. Cobalt is shown to have the ability to fly like Astro does.

The second Anime plotline shows that Cobalt was made before Astro by Doctor Tenma as a prototype, but that he was "imperfect" and was placed into storage. Professor Ochanomizu accidentally overlooked a cleaning schedule at the Ministry of Science and Cobalt was dumped into a scrap heap. Cobalt awakes and right away stumbles into all sorts of mischief. He's shown as a bit dim-witted when an elderly lady gives him instructions to the Instititute of Science and he can't remember them. He's also slow to catch on when people are bad and using him. In the Anime, Cobalt isn't shown to have flying capabilities like Astro does.

Appearences
Colbalt never appeared in the 1980 or 2003 series.
 * 1960 TV series