Overview
The characters in the Mirror Universe are generally the same as the characters in "normal" Star Trek continuity (for example, it has a James T. Kirk and a Spock), but their personalities are, on the whole, much more aggressive, mistrustful, and opportunistic. Whereas the Star Trek universe usually depicts an optimistic future in which the Earth-based United Federation of Planets values peace and understanding, episodes set in the Mirror Universe show the galaxy to be marred by continual warfare, with compassion seen as a liability. Uniforms are often more suggestive, such as women baring midriffs, and men with tighter pants.
In the 23rd century, during the original series of Star Trek, humanity had enslaved dozens of alien worlds and formed the Terran Empire instead of the United Federation of Planets (as exists in the main Star Trek universe). Officers were barbaric aboard Starships and discipline was enforced through torture — crew members carried agonizers and ships were fitted with agony booths. Officers could advance rank by killing superiors who they thought were incompetent. Roman/Nazi-style military salutes were used by all crew members to show loyalty. When exposed to individuals from the normal universe, the Terran Empire began to reform itself for the better but was overthrown in the 24th century as a consequence and replaced with an equally barbaric Alliance of several alien species who took advantage of the Empire's self-weakening to enslave their former masters.
Though the Mirror Universe is much darker and more violent than the normal Star Trek universe, a few Mirror characters are more friendly or docile than their "normal universe" counterparts. For example, Mirror Quark puts himself at risk to help escaped slaves reach freedom, and while "normal" Brunt is selfish and power-hungry, Mirror Brunt is a kind and considerate Ferengi.
There are an infinite number of parallel universes in Star Trek canon (as witnessed in the Star Trek: The Next Generation episode "Parallels"). However, the Mirror Universe is prominent among dimension-crossing incidents. The Mirror Universe has been visited in one episode of Star Trek, five episodes of Star Trek: Deep Space Nine, and two episodes of Star Trek: Enterprise, as well as in several Star Trek novels and several of the games (Star Trek: Shattered Universe, for example, takes place entirely in the Mirror Universe).
Not all "normal" characters have a Mirror counterpart; because the Mirror versions of Ben and Jennifer Sisko separated before conceiving a child, there is no Mirror version of Jake Sisko. Several licensed works also indicate that since humans (or Terrans as they are called in the Mirror Universe) were largely enslaved by the time period of The Next Generation, a mirror universe copy of Data was never created. Also, since the Dax symbiont was never joined with Ezri Tigan, there is no exact copy of Ezri Dax in the Mirror Universe, only "Mirror" Ezri Tigan. Several Mirror Universe versions of established characters have also died during the course of the various canonical and non-canonical Mirror Universe storylines, whereas their "normal universe" counterparts remain alive. The reverse is also true with Bareil Antos being alive in the Mirror Universe when his "normal universe" counterpart had died, and with Jennifer Sisko, who survived for many years after her "normal universe" counterpart died. (The Mirror Sisko eventually died herself in the episode "Shattered Mirror".)
Characters from each universe refer to the other as "parallel" or "alternate" rather than as "mirror".
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