(coexisting on another dimensional plane)
Professor Jennifer Sisko was a female Terran scientist of the 24th century Klingon-Cardassian Alliance and, later, the Terran Rebellion. Jennifer was married to Benjamin Sisko, but they separated in 2366. They did not have any children. (DS9: "Through the Looking Glass")
Sisko and Benjamin married each other in the 2360s, but Sisko became increasingly frustrated about Benjamin's attitude. She viewed him as needing to impress other women, and too devoted to his ship and crew to make time for her. She finally wrote him off when he left her to serve Intendant Kira.
Sisko disapproved of the Terran revolt that started in 2370, viewing them as killers with a misguided notion of attaining freedom. In particular, she felt that Benjamin, who became their leader, simply wanted to fight. She felt the rebellion only gave an excuse for the Alliance to use more force on the larger Terran population. She rather felt that the rebellion needed to be stopped for peace.
She found herself working for Intendant Kira on Terok Nor in 2371, when she was tasked with building a transpectral sensor array in order to find rebellion forces inside the Badlands. This made her a prime target for the rebellion. While the larger rebellion wanted to kill her, Benjamin wanted to convince her to join them. However, he was killed before he could try. Unbeknownst to her, a plot by Miles O'Brien brought another Benjamin Sisko from a parallel universe, and O'Brien convinced him to follow through with the plan. Sisko came to see him after he was captured on the station. She still hated him, but he seemed different than before, and so she was convinced to switch sides and join the Rebellion. She went with him and escaped to the rebel base. There, she found out he was not really her husband. (DS9: "Through the Looking Glass")
In 2372, Jennifer abducted Jake Sisko from Deep Space 9 in order to lure Benjamin Sisko back to the mirror universe. Her attempt was successful and Ben helped the Terran Rebellion to build a Defiant-class vessel for Terok Nor. While Jake was in the mirror universe, he and Jennifer grew close and became almost like family despite protests by Benjamin Sisko. While attempting to bring Jake back to his universe, Jennifer was shot by Kira Nerys, who had escaped Terran imprisonment. (DS9: "Shattered Mirror")
When Kira Nerys encountered the Bareil Antos of the mirror universe in 2374, Sisko told her it had been confusing meeting Professor Sisko because, despite knowing she wasn't his wife, he had almost come to believe she was. (DS9: "Resurrection")
In the first draft script of "Through the Looking Glass", this character was called "Jennifer Hall", rather than having the surname "Sisko".
In a pitch letter that the DS9 writing staff sent to freelance writers prior to season four of the series, stories about the mirror universe version of Jennifer Sisko was a form of narrative which the staff writers listed as having no interest in receiving. An in-house plot featuring her turned up shortly thereafter. (Star Trek: Deep Space Nine Companion, pp. 191-192)