Captain Atom may not be one of the most well known DC Comics characters, but he is arguably one of the most powerful. He is a being of pure nuclear energy contained within a suit. Because of that, he is able to fly, have so superstrength, and can shoot and absorb any kind of nuclear or radioactive material.
His personality varies depending on the show, but often he is depicted as a tried and true solider. A man who follows orders and fights for what he believes in. This is what made him a valued member of the Justice League, and an ally to Batman.
Also though, he's never afraid to risk his life to save people or the world. Though as people find out, he's harder to kill than they realize.