4. Dr. Phlox
In the Prime timeline, Dr. Phlox is the Enterprise's Denobulan doctor. He's a curious, quirky, and optimistic being, which is what makes his Mirror counterpart so frightening! There, he's sadistic and cruel, as his natural curiosity is twisted into something awful.
It leads Phlox to perform terrible experiments on the creatures his counterpart treats with care. He even delights in torturing and killing a Tholian once he and Archer extract the information they need to find the USS Defiant, a starship the Tholians had captured from the Prime timeline.
While Phlox has several wives in the Prime timeline, as is Denobulan custom, his Mirror double is ultimately driven by T'Pol's suggestion that the Empire will reward him with unlimited resources and concubines, something that sways the doctor to join her rebellion against Archer.
This suggests that Mirror Denobula is likely similar to Ferenginar in Deep Space Nine, where historically, women had few rights and men ran everything. This is one of the attractions of a mirror universe, because, as optimistic as Phlox is normally, his counterpart's personality is so terribly cruel and immoral! And don't forget, Phlox was one of those responsible for creating the Mirror Universe's infamous agony booth.