I would actually argue that Lucina does in fact share the main character spot with them, she just tells a different side of the story
She's a much smaller plot character overall. The main stories focus around Chrom and then Robin. Lucina has her part too, but I wouldn't say it's nearly as important.
As I've said in past things, there's multiple main characters. She's still a main character, but more of a secondary one. It wouldn't affect Smash either way, since Chrom depends upon Roy returning. Unless all Clones and Echoes are cut, there's little reason to believe Lucina would go at any point. The only Echoes that might get cut otherwise is 3rd party negotiations falling through. Most of them are too easy to make anyway(and one barely has a gameplay change. 0.000001% of differences is pretty awful. Sure, the Belmonts aren't much better, but jeez. You can do a little more than that. Dark Samus, imo, for one who only has one elemental difference, is a good bare minimum change-wise. Since there's lots of stuff that affect electric vs fire. That, and the animations make her seem a lot more different than she is. Chrom and Ken are great for other reasons too, and Lucina has no tipper, which is a pretty important gameplay change. Dark Pit's a little eh, but he does have some different moves, so the strategies slightly change from what I remember).
I did misspeak. By not calling her a main character earlier. My bad.
I've made some notes on Super Mario RPG too, since it has a lot of main characters.
Mario and Geno: The core plots also revolved around both. Geno is introduced later and is decently comparable to Robin, that is, not the starting main character but much important. Saving the world is the first plot. Saving the star road is the even bigger plot overall.
Bowser: The core plot has him hard involved, but for reasons that don't matter much at all. He's not however like Lucina, since it leads to nothing important. He's just a cool villain given a fun plot.
Mallow: He holds a slight bit more importance than Bowser due to actually having his plot lead to a tie-in to the main plot in a meaningful way(he isn't there for the ride as an ally). That, and it's fed to you almost immediately, so it is kept throughout the story. This is more comparable to Lucina. A side plot to some degree, but actually more important than it seems.
Peach: Lol, her plot is "give an excuse to start the game off at first, as joke to lead into the first of the two core plots". So more or less what she usually does. Gets kidnapped to make the game "start". She has no significance to the main plot after that. She is still at least put into the story nicely with a boss arc, at least. I mean, I wouldn't say it's the best-written idea, since she's still being heavily treated like her only notable role is damsel in distress, but it's like her original plot point so a plot-driven rpg using it isn't odd. I'm just glad she's existed for reasons beyond that(Zelda as a character has gotten it a little better, but not much).
