We think you will find this next individual rather different from the usual mystery-man type.
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Powerman IV

August 1958
    The mightiest team in the world is the partnership of Superman with Batman and Robin, the Boy Wonder. Time and again they have joined forces to meet a great crisis, until one day a mysterious new hero comes between the world's finest team. This is the story of Powerman!

    One night in Wayne Manor, Bruce Wayne and his young ward Dick Grayson, while watching a newscast, learn of Lex Luthor's escape from prison, which they know means trouble ahead. Luthor has boasted that some day he would get his revenge against Superman and Metropolis, the city he protects. Deciding to aid Superman, Bruce and Dick enter the Batcave under the mansion and swiftly change into Batman and Robin, the Boy Wonder! Robin tells Batman that Superman may need their help if Luthor uses kryptonite, which weakens and can even kill Superman, to seek his revenge.

    A swift ride in the Batmobile brings them to the outskirts of Metropolis, where they rig up a Bat-signal to attract Superman's attention. Soon he arrives with a mysterious figure, Powerman. Batman offers to help, but the Metropolis Marvel, though grateful, explains that this is unnecessary; Powerman will help him if he needs it. Powerman declares that he doesn't want any more people on "their" team--he and Superman can handle this job by themselves. The masked stranger then tells Batman and Robin that he and Superman must continue searching for Luthor without wasting any more time.

    Once Powerman and Superman leave, Robin comments on Superman's odd behavior, implying that perhaps Powerman has some hold on Superman. Batman concurs, noting how domineeringly Powerman spoke to Superman. The Dynamic Duo decides to follow the Titanic Two on their search for Luthor. Hiding behind a lead ornamental cornice, Batman and Robin watch Superman comb the area with his x-ray vision, while Powerman searches on foot. Superman spots a gas leak from a loose main under a building, noting that the building will explode with just a spark. Superman lifts the fortunately deserted building up to let the gas dissipate, while Powerman tightens the loose pipe. Seeing this feat, Batman realizes that, while apparently not as powerful as Superman, Powerman has super-strength.

    Batman and Robin continue to pursue Superman and Powerman. Superman spots Batman and Robin following him, and he and Powerman confront the Gotham protectors. Batman is concerned that Luthor may use kryptonite against him. But Powerman declares that HE is the only help Superman needs and, that with Luthor’s hideout so close, Powerman doesn't want them around. Superman agrees and orders them to leave the scene. Batman and Robin head for the Metropolis prison to investigate Luthor's escape. They find silicon traces left by a destructive ray in the blown-out wall of Luthor's cell. Batman concludes that this ray, coupled with kryptonite, could harm Superman. But Batman has figured out a way to help Superman.

    He finds Superman and his new ally at Luthor's hideout, where Lex is going to use the destructive ray to take his revenge on Metropolis and Superman. Superman is about to confront Luthor, but Batman intercedes. He and Robin have constructed silicon shields, which immunize them against the deadly rays. Thus, they can charge in on Lex and take away the kryptonite, while Superman captures Luthor and his gang. Mission accomplished, the three of them are reunited. But Robin wonders why Powerman did nothing. Superman explains that Powerman was really nothing more than a robot, an automaton with a built-in tape recorder that Superman could activate via remote-control to make Powerman speak.

This was the mystery of Powerman!