The Skeleton was an unidentified person with considerable hatred towards Bruce Wayne. After the Earthquake that turned Gotham City into No Man's Land, he discovered the various underground layers of Gotham's most notorious criminals and appropriated their leftover costumes and equipment. The Skeleton studied the villains and used special equipment so that he could physically mimic several of them. As the Joker he stole deadly toxins from a chemical plant, as Riddler he threatened the life of a secretary, and as Mr. Freeze he attempted to restructure the city's power lines and turn Gotham into an inferno. The crimes he committed were done to direct Batman's attention away from acts of sabotage that were securing the Skeleton's actual goal of sullying Bruce Wayne's reputation. This would serve as the initial phase of Wayne's annihilation.
The Skeleton was introduced by Brian K. Vaughan in Skull Duggery, a 5 page short story that appeared in Batman Gotham City Secret Files 1. The story itself introduced the character without any actual confrontation with Batman. The Skeleton never appeared again. Vaughan teased the idea that the Skeleton was someone Bruce Wayne was familiar with but nothing was ever revealed.