Star Trek: 10 Reasons With Sisko & Kira's Relationship Is Key To Deep Space Nine
2. A Worthy Successor
Part of a First Officer's job is to be prepared to take over in the event that the Captain can no longer command. It's necessary for the First Officer to know as much about the operations and all other aspects of the ship, or station in this case. Fortunately for Sisko, he had that in Kira.
At different points during the series, Sisko was away from Deep Space Nine for an extended period. One such time was after the death of Jadzia Dax. He blamed himself, and retreated to his father's restaurant in New Orleans. While he was there, Kira kept the station running smoothly.
This became even more important at the end of the series. The finale featured the final battle between Sisko and Gul Dukat, the respective avatars of the Prophets and the Pah Wraiths. Sisko won but never left the battlefield. He didn't die but was taken to the Celestial Temple by the Prophets for an indeterminate amount of time.
Thankfully, Kira was there to take over the station and maintain the continuity of everything she and Sisko had built together.