Robert Trebor is an inventor from a neighboring mirror universe who comes to Callahan's via a mirror installed behind the bar. He introduced the patrons to Wonderbooze and wanted to trade it for rotgut, which they couldn't understand why he would want to do this.
His motivation for doing so was revealed when he was discovered to be a fugitive from another universe. He had brought the alcohol with him from there. Crossing into this universe changed it from rotgut into high-quality alcohol. Likewise, rotgut from this universe would become Wonderbooze in his, thereby making him rich.
He was caught after the FBI from his universe was able to pass directions to the patrons in this universe via the mirror behind the bar to send him back. He had panicked and given himself away when he glanced at the backwards clock Mike Callahan hung on the wall, thinking he was back in his own universe.
Background information[]
The name "Robert Trebor" is a palindrone, a word or phrase that is the same spelled forwards and backwards.