lightofdaye wanted a compare and contrast of Ezri Dax (DS9) and Elizabeth Lochley (B5).
Let's start with the obvious: what Ezri Dax and Elizabeth Lochley primarily share is that both characters were last minute replacements for show regulars in their respective show's final season, who came into existence because things with an actress playing a series regular did not work out for various reasons. As both characters had been very popular - Ivanova even more so than Jadzia Dax (which reminds me, I do wonder what would have happened if Nana Visitor had been the one to leave! Because DS9 without Kira is unthinkable...). This alone ensured a considerable part of fandom would resent the new characters, irrespectively from how well or badly they were written and acted.
Now, it's interesting that both shows used almost diametrically opposite narrative techniques to deal with this inheret drawback. DS9 went out of its way to establish Ezri as not simply being a Jadzia clone and devoted considerable screen time to her. She was given a different job (counselor to Jadzia's science officer, which reminds me, I don't think we ever found out who replaced Jadzia as science officer?). We got to know her pre-Symbiont family, the Tigans, and their background in an episode that was all about them. Where the last episode that had been Jadzia-centric without also being Worf-centric had been several seasons ago, Ezri in addition to the episode exploring her birth family got another Whodunit episode focused on her as the detective, this one co-starring former host Joran as Hannibal Lecter (it was very much the age of Silence of the Lambs) and an episode in which she had to counsel and prove herself to the show's most popular recurring guest star, Garak. There was a continous, unchanging relationship - the friendship with Sisko - and one relationship that turned out to be quite different from the one Jadzia had, with Worf, but had its commonalities and differences explored in great screen time detail. (And then there was the last minute romance with Bashir, which got so little screen time to develop that it never felt like the writers were either interested in it or believed in it.)
Meanwhile, JMS went almost the opposite way with Elizabeth Lochley. Technically, she got Sheridan's old job, not Ivanova's, but it was evidently the job Ivanova would have had if Claudia Christian had not left the show. There was no Lochley-centric episode to introduce or explore her the way there had been when Sheridan had taken over as station commander in s2; instead, she got introduced through the telepath story arc on a professional basis and through the confrontations with Garibaldi on the subject of her loyalties on a personal one, until the one episode not written by JMS but Neil Gaiman, Day of the Dead, which did more to flesh out Lochley's character and background than anything that had come before (as JMS himself freely admitted in the preface to the published script). The one relationship with a regular she was established to have had before her introduction to the show, with Sheridan, got as little screen time as possible; it's almost till thet last but one episode that the two of them share a scene and conversation which isn't about the plot of the week and feels a bit personal. Where Ivanova had been given key parts in the show's political arcs (both in the Shadow War and the Earth Civil War) as well as two popular romantic relationships (with Talia and then with Marcus, Lochley during season 5 had no screen time devoted to her personal life (this changes in Crusade, but we're talkingn about B5 here), and while she had a role to play in the telepath mini arc that dominated the first half of the season, she had no part in the Drakh/Fall of Centauri Prime arc dominating the second half. Her main scenes in the second half of the season involve Garibaldi's storyline and are a great pay-off to the hostility between them established early in the season, but they're still mainly about Garibaldi, not Lochley. In short, I'd say s5 goes out of its way to signal to the audience that Lochley isn't taking anything away from beloved regulars; she's the unexpected houseguest who keeps to herself most of the time, not the one insisting on sharing every conversation and choosing what's on the menu for dinner.
There are pros and cons to either approach. For example, my guess as to why JMS went out of his way not to give Lochley and Sheridan any one on one scenes together that aren't about station business is that he wanted to avoid even the whiff of a suspicion he was setting up a love triangle here between Sheridan, Lochley and Delenn, which in a season where there's already an emotional triangle between Sheridan, Delenn and Lennier, fair enough. (Not to mention that Sheridan/Delenn was the most popular relationship on the show, and Lochley really didn't need any shippery resentment directed her way.) Still, that meant, and my most recent B5 rewatch underlined this for me, that the Sheridan & Ivanova friendship, which had been one of the most endearing platonic m & f relationships on the show and brought out the best in either party, had not only disappeared along with Ivanova, but that Sheridan now shared personal screentime only with Delenn, what with Garibaldi having his Bester-caused off the wagon arc and Franklin also not given to hanging out with Sheridan, while Londo and G'Kar were busy with the unfolding tragedy on Centauri Prime, so the effort to not let Lochley intrude might have actually backfired and robbed Sheridan of something of his human warmth. Equally: given that one of the key ways in which DS9 made Ezri different form Jadzia was that Ezri had ended up with the Dax symbiont accidentally, had not been prepared for it and had not originally wanted the symbiont life, it made narrative sense to let the audience see Ezri Tigan's family and have her interact with them to explore whether and how Ezri Dax was different from Ezri Tigan. I have my problems with that episode, but not with the basic premise. Just as, to look at another Ezri episode, the idea of Ezri facing the challenge of playing Counselor to Garak, of all the people, is a good one - can't ask for a bigger challenge - but the execution...
All of this, btw, still didn't stop me from liking both characters. I thought making Lochley someone who'd been on the other side of the war was an interesting premise, as was her different relationship with Bester. She had great chemistry with Garibaldi, and both their initial hostility and the way she ended up helping him really worked for me. Day of the Dead, and the reveal of Zoe, the story of Lochley's youth and how it still haunted her was great. And I took the additional tidbits we were given about Lochley - for example, that she speeks a little Centauri - and used them in fanfic. And I loved all three of her Crusade episodes. Meanwhile, Ezri's scenes in the opening s7 three parter are just what the doctor had ordered to get me and Sisko out of post-Jadzia gloom, Nicole de Boer had a knack for projecting likeability, and the story with Worf worked on a "exes who don't get together again because they realize they're (literally) different people now, but people who know each other still really well and thus can be each other's confidants" for me. If there's ever a Star Trek: Sisko show the way there was Star Trek: Picard, I do want to meet Ezri again and find out who she has become in the intervening decades. Especially without the writerly pressure of having to present her in a last season situation.
In conclusion: not having written a multi season tv show set on a space station with a big ensemble and multiple storylines, I'm not sure what I'd have done when faced with the need to replace a popular character with only one more season to go, whether I'd taken the Ezri Dax or the Elizabeth Lochley solution, and whether all those things I as a watcher concluded should have been done differently would have occured to me as a stressed out writer. But these are my thoughts on a character comparison.
The other days