Cobalt

 is Atom's younger or older brother depending of the version which he appears, however he is a non recurring character as he doesn't appears in the series remakes and is not quite remembered.

1952 manga séries:
Cobalt was created by Dr. Ochanomizu after Atom went missing while a H-Bomb was about to detonate and he had no time to look for him

However, due the fact Dr. Ochanomizu was rushed by the other scientists he could not create an advanced robot model like Atom which resulted into Cobalt lacking all of his missing brother's abilities and also lack his inteligence.

Cobalt was unable to detect the H-bomb, but he managed to find and track down Atom's location who decided to de-activate the H-bomb, but had little energy so Cobalt shared his own energy for the job and Atom managed to shutdown the bomb.

Later a group of russian scientists found both Atom and Cobalt and also gave the promise of repairing Cobalt while they actually wanted to study Cobalt and copy his technology to improve their own robot model, but as Cobalt himself was not so advanced, their plan backfired.

Cobalt later took minor roles which he did few cameo appearances, but in Midoro Swamp, he sacrified himself in order to defeat the enemy robot created by the Dinossaurs who wanted to destroy humanity, but Tezuka later changed the story making him survive.

He later re-appears rebuild at the end of the original version of "The Invisible Giant" when Uran is introducted, though in the re-issues of the original mangá, his death was omitted entirelly.

1963 series
The basic plot of the manga is used for the 1963 anime, however, Cobalt is now despicted as a failed prototype found by Dr. Ochanomizu in a scrap heap.

Which make Cobalt become Atom's older brother as he was also created by Dr. Tenma.

Trivia

 * Cobalt is been excluded from the remakes due the fact Atlas was recasted as Atom's brother in the remakes, starting with the 1980 series.
 * However, it's possible that Tezuka had lost interest in Cobalt mostly due the fact he was just like Atom himself and also lacking his abilities.
 * Cobalt is excluded from the 2003 anime series, regardless if Atlas himself doesn't consider himself as Atom's brother since he have Daichi Tokugawa's mind. The dub of Atlas' story is non canon.
 * Cobalt was excluded from the manga stories of Tetsuwan Atom made in 1980, despite the fact Atom and Atlas were not brothers, a sign that Tezuka might had lost interest on him.
 * Cobalt is mentioned in "Astro Boy - Tetsuwan Atom: Atom Haato no Himitsu" (called overseas as "Astro Boy: Omega Factor".
 * Cobalt's origin as a prototype in the 1963 series have a continuity issue, as it's pretty unlikelly that Dr. Tenma and his assistants were building him in the first episode instead of Atom. Also it's rather unlikelly that Dr. Tenma had him as his first child like robot, considering that Dr. Tenma's assistants thought he was out of his mind while wishing to create a child like robot which turned out to be Atom.
 * Cobalt look was pretty alike Atom himself in the manga, however he got taller and also got slightly different from Atom and the deceased Tobio Tenma in the 1963 anime.