Super Smash Bros. Ultimate director Masahiro Sakurai has commented on the wide range of Fire Emblem characters in the fighting game. Sakurai even acknowledges complaints that there are too many Fire Emblem characters in the game, but he's not keen to dwell on it.
"There are too many Fire Emblem characters," Sakurai writes in his Famitsu magazine column (as translated by regular Famitsu translator Ryokutya2089). "I, too, understand. However, these things are already decided, so we move forward. The things I should be thinking about are elsewhere."
There's undoubtedly been a backlash to Byleth's inclusion as the fifth and final DLC character added to Super Smash Bros. Ultimate through the game's Fighter Pass. With Ike, Marth, Chrom, Lucina, Roy, Robin, and Corrin already on the roster, some felt like adding in Fire Emblem: Three Houses protagonist Byleth was a bit unnecessary, and they would have rather seen a character from another Nintendo franchise.
Sakurai also writes about the process behind introducing Byleth to Super Smash Bros. Ultimate. "I don't choose the fighters based on my own preference, that's for Nintendo to bring in," the director writes. "This time around, we brought the experience of immediately playing as the protagonist that people currently play. At the starting point of its production, we experimented by working with a title that was in development."
It's interesting to get this time frame of development for a new Smash Bros. character from Sakurai. From the director's comments, Byleth was in production for Super Smash Bros. Ultimate before Fire Emblem: Three Houses launched in July 2019. That should give you a rough idea of how long it takes to develop a character as complex as Byleth for a fighting game (yes, Byleth is more complicated than "just another sword user.")
Byleth was the fifth and final character for the current Fighter Pass, but last month Sakurai revealed DLC Fighter Pass 2, which is now available to purchase. Neither the director nor Nintendo has revealed the characters set to feature in the pass, but we do know that they've already been decided, and there'll be six new characters in total.