The mirror-universe Benjamin Sisko is a swaggering Terran privateer who collects tribute for the Intendant until an encounter with visitors from the prime universe pushes him into open rebellion.
In the mirror universe Benjamin Sisko is one of the few Terrans allowed a measure of freedom. He captains a raiding ship, gathers duties for Intendant Kira and enjoys her favour as a lover, a comfortable arrangement he describes as the best a human can hope for.
In Crossover he meets the prime-universe Kira, who challenges him to do more for his people. At the end of the episode he turns on the Intendant, helps Kira and Bashir escape and declares his intention to stir up trouble for the Alliance.
By the time of Through the Looking Glass (2371) he has become the leader of the Terran rebellion and has been killed in action. The rebels kidnap the prime-universe Sisko to impersonate him and complete his final mission.