First of all, this is a countdown that demands a few honorable mentions.
If you haven't seen it, here are the basics:
After the planet Bajor at last repels a generations-long occupation by the Cardassian Empire, The Federation of Planets sets up an outpost in orbit around Bajor called Deep Space Nine, at the request of the Bajoran Provisional Government, to help with reconstruction and monitor Bajor's progress toward qualifying for Federation membership.
Kira Nerys, a Major in the Bajoran Militia, is assigned to Deep Space Nine as Federation Commander Benjamin Sisko's First Officer, in the hope of encouraging Bajoran/Federation cooperation.
Kira's been fighting for Bajoran independence since childhood, and she's not thrilled about depending now on a more powerful ally. She and Sisko immediately get off on the wrong foot, and to make things more complicated, a stable wormhole connecting the Alpha and Gamma quadrants of the galaxy is discovered next to Bajor shortly after the establishment of DS9, making what was supposed to be a backwater outpost into a hub of scientific exploration and, like it or not, a critical strategic position.
She's a guerrilla freedom fighter suddenly turned legitimate by her people's victory over their war-crime-happy one-time conquerors, and that equation isn't glossed over.
She's a passionate, uncompromising defender of the downtrodden, but she's got plenty of moral grayness, some war crimes of her own under her belt, and the concept of fun tends to elude her after all the unimaginable awfulness she's been through.
Hey, it's still Star Trek.
Over the course of the series, Kira has to become more open-minded, form friendships across cultures, make an uncomfortable transition from violent rebel to administrator and diplomat in order to keep advocating for her people, and allow herself to recognize corruption where it arises among the Bajoran clergy, all without losing sight of her personal faith.
They each get their own in-depth plotlines, romantic and otherwise. They each get to play protagonist in a nearly equal number of episodes, and while his tie in more closely to the (not particularly interesting) overarching mythology of the series, she becomes more of a leader at the station than he is with all his mythology related distractions, and her plotlines are more frequently action-packed and emotionally powerful.
She’s the one who ends up forming a close friendship with an elderly Cardassian man after being kidnapped and forced to impersonate his daughter.
She's the one who ends up sharing guardianship of a naive political refugee with the same Cardassian overlord who abducted Kira's mother as his consort when Kira was three.