First off, as I said before, I didn't find MU Kira recognizable at all apart from the fact that she's played by the same actress.On another level, Crossover rejects the notion that our heros would remain fundamentally unchanged no matter the circumstances (as is the case with Spock). While Kira, Odo, Sisko, O'Brien and the others are all recognizable, they have all been deeply impact in one way or another by their experiences in this darker, more violent universe.
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Second, I don't think that "Mirror Mirror" ever suggested that our heroes would remain unchanged no matter what the circumstances. Kirk commits genocide, assassinates Pike in order to replace him as the captain of the Enterprise, kills all his enemies; Chekov tries to assassinate Kirk; Sulu is a leader of a Gestapo-like security force and sexually harasses Uhura; and Spock tortures people, threatens them, forces mind melds on them, and is ready to assassinate Kirk if given the order by Starfleet command. I don't know where people ever got the idea that Spock is unchanged in the Mirror Universe. Must be the consequence of the ending of the episode where he is influenced by prime Kirk & co. decides to change the Empire for the better. By that logic, Sisko is fundamentally unchanged in the Mirror Universe because he starts the rebellion at the end of "Crossover". Yes, MU Spock is not personally ambitious and he is honest about his intentions, but that is because he is still Spock, and he has to have some similarities to the prime Spock beyond the looks, in order to convincingly be the same person from an alternate universe. You can just slap a bunch of random characteristics and say that it's an alternate version of the same character, if they're not similar at all. Which is why Mirror Spock is, IMO, the example of how Mirror characters should be done: he is recognizable as Spock, but he is different, he's ruthless as one would have to be in the MU universe.
edit: Now I see that you've edited that part from your post, so it seems like I'm replying to a non-existent post.
