Okay!

For starters, in the manga, He was built as a replacement for Astro when he ran away, feeling upset when everyone kept on coming to him with their problems. He had it when they asked him to defuse a bomb. When Cobalt was built, he looked perfectly like Astro, but couldn't perform detailed tasks; so in other words, he couldn't defuse the bomb. In the end, Astro came back and defused the bomb, and I don't know much else. :d oh:
In the 1960's anime, he had his debut in Episode 84: Brother Jetto. He was actually built before Astro, kind of like a prototype. Tenma threw him out when he found out that Cobalt was imperfect. One day, Ochanomizu had shown Astro picture of him, explaining how Tenma created him and threw him out. Astro wondered out loud if he was still in the scrap area. Ochanomizu then told him that he cleared out that area, providing some extra space. Astro started worrying if Cobalt wasn't deactivated, and went to find him. Meanwhile, Cobalt had already awoken, trapping a bird under some scrap metal. He freed the bird, asking to be friends, but had been denied. I'm going to stop here, so that I don't get carried away rambling, or go too off topic.
