Cobalt originally died a heroic death at the conclusion of the Midoro Swamp story arc in the Shonen Magazine serial version of the manga, but Tezuka later changed his mind & rewrote the ending for the paperback version so that he survived.
The Anime, (on episode 84) 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 Institute 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.
Cobalt was first introduced in Chapter 21 "Cobalt". In the chapter, after Astro disappears, Professor Ochanomizu made a rough copy of him to defuse a bomb. However, Cobalt's electronic 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.
- 1963 TV series
Cobalt never appeared in the 1980 or 2003 series. Though he is referenced in the 2003 series in the fact that The Blue Knight's real name was "Cobalt."