For all we know, this character could have just been bumped to the second pass, perhaps even the reason why FP2 has six characters. Maybe when it was decided to have a second pass, they just thought to change up the order. Funny enough, that would strengthen the 'Byleth wasn't character 5' theory.
Sakurai said the DLC list for pass 1 was locked back in late 2018, at a time when pass 2 wasn't a thought. At no point has it been admitted that after that things changed. So one theory is that Nintendo confirmed list 1, saw Ultimate be a huge success, decide on pass 2 but mix around the remaining unknown characters' order.
I dare claim, with no evidence or science, that Byleth WASN'T a part of pass 1 in 2018.
Honestly the second dlc pack was probably thought of at the same time as the first.
I wouldn’t go as far as to say the dlc pack 2 wasn’t a thought. Like most AAA titles there is probably some kind of road map they are following that illustrates where they see the game in like the next 5 years, while also keeping in mind releases for other games as well as the consoles life span.
Sure Sakurai is the face of the development team for this game, but it’s unlikely that they operate independent from Nintendo’s own plans.
I would agree that the second wave of dlc probably wasn’t as concrete as the first, but there were probably already a handful of characters that already populated a potential list. And the list is probably subject to change as well.
As far as how characters are picked, I would assume its more of a back and forth thing. Nintendo and Sakurai comprise a list of characters. Sends Sakurai a list to look over. Sakurai tells them what he thinks would work. Then Nintendo essentially has the final say as to what characters are “necessary“.
Overall if anything my assumption is that the success of the first pass didn’t spark the thought,”hey let’s do another pass” , it just green lights a plan they already had in place.
There are multiple Dlc mii costumes still missing, which isn’t concrete evidence by any means, but helps support the idea that Nintendo had already planned to sprinkle these pre existing mii costumes across multiple packs. Because for the life of me, I can’t see why we’re still getting dlc content that was released before. Another assumption, but it’s probably because from a consumer perspective it makes it seem like we’re getting more content than what we’ve previously gotten making it a more enticing buy.