One of the strongest beings in the DC Comics universe, the original Captain Marvel was at one time the best selling comic book hero, even beating out Batman and Superman, then some unfortunate things happened and he was forced to leave comics.
But you can't keep a good captain down, and he made his return to DC Comics, and he hasn't left since. Billy Batson is a young orphan boy with a heart of gold, because of this, he was chosen by the wizard Shazam to wield the powers of the pantheon and become his champion of justice. When Billy cries out the wizards name, he becomes Captain Marvel.
Fighting for justice for all the right reasons, Billy is a trusted member of many teams, including Batman's Justice League in Brave and the Bold, and has been a member of the proper Justice League many times.
Because of his age, Billy is often naive, but he just uses it to his advantage, and it makes his opponents underestimate him. Batman has nothing but respect for Captain Marvel, and he knows he calls upon him for help, Captain Marvel won't hesitate to assist.