Lyn, the clear-cut character people are begging to break into smash... is kinda minor in the grand scheme of both the franchise and even the original game she comes from.
(spoilers for FE7)
Well, the main bulk of FE7 is technically Eliwood's story, yeah, and Lyn was created to be the main character of the "tutorial" portion. But in practice, from the perspective of a first playthrough (which will be all that the vast majority of players will do), Lyn is unequivocally the main and by far most important character for the first 10 chapters. And even if a lot of those chapters are pretty small in comparison to the later ones, it's still a significant deal of narrative effort expended on specifically her, full of moments of her speaking directly to the player and complete with its own CG credits roll. It's enough to really skew the optics in favor of her, especially for newer players. And then, it's not like she disappears completely in Eliwood mode. She's back in the picture by chapter 15, getting two chapters about saving her granddaddy, and from there continues to appear in basically all of the cutscenes and remains involved in all the party's decision-making alongside Eliwood and Hector. And, while the initial arc of Eliwood's story is motivated by the search for Eliwood's father, once they find him and everything is about stopping Nergal, all three of Eliwood, Hector, and Lyn are pretty amply motivated, albeit with most of the emotional weight tied to Eliwood through his relationship with Ninian and the loss of Elbert.
Overall, it's super easy to see why players would come away from FE7 loving Lyn and either not caring or, honestly, not even noticing if a lot of the story is technically about Eliwood, especially newer players (of which there were a ton for FE7, it being the first game released in the West). Regardless, I don't think it's fair to call her a minor character when, even if you want to say Eliwood is the most important, she's pretty easily argued as the next most important. And of course that's only strictly plot-wise, because she trumps Eliwood hard in popularity, getting the spotlight in the opening segments of the first non-JP game has led her to a greater overall notoriety than him among fans, and of course she is the one getting all the spinoff highlights and etc.
In terms of significance to the series at large, well, pretty much nobody in the series gets to make a major, main-character-level impact on more than like 2 games, with Marth and Ike being the only ones who get to be true main characters for multiple games (Alm and Celica if you count the Echoes remake), and even Ike is taking a backseat for significant portions of RD. So unless the argument is just that all Fire Emblem characters are ultimately unimportant to the grand scheme of the series, being in one game isn't really hurting her ranking very much.
Her importance is essentially tied to two things, being a poster child for FE's break into the West, and being absurdly popular, with only Ike really standing over her (and even then it's actually fairly close). With both those elements to her status having been confirmed to be recognized by devs.
Though if the point here was just to compare her to Tiki and Anna, it's no contest. Not in terms of plot importance or popularity.