Bradward Boimler was a hypothetical mirror universe version of Ensign Brad Boimler depicted in the Mirror Universe Encounter holoprogram.
In 2381, when a drill administrator named Shari yn Yem was putting the crew of the USS Cerritos through various training drills in simulator holopods, Ensign Beckett Mariner started the program, and found herself playing the role of an officer who had taken the place of her Terran counterpart.
When she encountered Boimler, he was laughing in an evil manner, stating how he was going to have to come up with a plan to either impress or assassinate the ship's captain.
When Mariner did the Terran salute, Boimler immediately noticed that she was using her left hand, instead of her right hand, and so he suspected that she wasn't the Mariner from their universe. He immediately had her seized. (LD: "I, Excretus")
This hypothetical individual was voiced by Jack Quaid.