A "masked marvel" was a fighter who would enter the ring wearing a mask or other disguise, which shielded his identity. Most appearances of the Masked Marvel were announced prior to the show, in newspaper articles and advertising.  After he entered the ring, he would remove his mask and reveal his identity to the crowd. Usually the boxer was well-known to regular attendees of local shows.
The use of the masked marvel was much more common in wrestling, where the element of surprise has long been used as a promotional device.