Before the announcement, I'd pinned Byleth as one of those characters that people had arbitrarily determined were as good as confirmed, but truly wasn't (in the same vein as, say, Dante, Crash Bandicoot, and Cinderace--all characters that discussion circles treated as guaranteed, but turned out to be untrue). I thought it was actually unlikelier than most, given how recent the game was and how much Fire Emblem representation Smash had already seen. I put it in the same bin as those expecting a Gen VIII Pokemon character, both during that round of DLC and the second. Because the logic was solely "A new game came out recently, so that means they must add a new character" with either no further reasoning, or at the very least "because Nintendo wants to advertise their newer games," when the latter almost never happens.
Especially when DLC characters seemed to be more focused on using these unique DLC opportunities for adding new series and worlds to Smash, so saving a character from one of the already most represented series in Smash would obviously be disappointing in a general sense. And after all, even if we were to add more to a Nintendo series that already existed within the game, you have Star Fox, Kirby, Donkey Kong, and Zelda that haven't seen new characters since Brawl and are far more popular than Fire Emblem. There's plenty of unique and untapped potential in those series, so they'd surely get consideration before Fire Emblem.
Needless to say, I was, and still am, confused by Byleth's inclusion. On a gameplay level, I can see how their moveset is unique and has been used to great enjoyment by many players. But on every other level, it doesn't make a whole lot of sense, except for the acknowledged fact that Sakurai and other staff on the Smash team are big Fire Emblem fans.
For those who enjoy and were happy for Byleth, more power to you. But out of the 11 total characters added as DLC, their inclusion is the weakest by far, relative to every other addition that either added a completely new series to the game or expanded upon a series already in the game that actually needed it.