Dave the Dummy was a former member of Catwoman’s gang. He was later disguised as a juror by Lucky Pierre during Joker and Catwoman’s trial.
The Joke’s On Catwoman
Dave was disguised as Juror Number 2 by Lucky Pierre in order to make sure Joker and Catwoman got off. When the Jury Foreman informs the judge of their ‘not guilty’ verdict Dave pats him on the back in a congratulatory way. However after the disguise of the Jury Foreman, aka Marvin the Moose, fell off, he and Marvin were both recognised by the Caped Crusaders. In the ensuing Batfight Dave was captured.It is likely that Dave has low intelligence, judging by his nickname.