Sidekicks (The Enemies Trilogy Book 3) - The Enemies Trilogy, #3

In the thrilling conclusion to the Enemies Trilogy, not even superheroes are bulletproof. To keep the legend alive after Good Guy is murdered, his sidekick Ajax Gadzinski is asked to put on the mask. Only Ajax is more interested in avenging his friend than joining the long line of heroes who died for the cause. Besides, if the rumors are true, the original Good Guy is back, saving people like he never left.
Or, you know, it could just be the evil twin.