Actually, let's expand out on Byleth because I am relatively certain that Byleth is the only objectively bad thing about this game. I don't care about spoilers anymore, this game has been out for over a month and this community has picked it clean within the first two weeks of its existance.
So let's start with the "Rewind Time" feature which is the least egregious sin of Byleth. And I'll be the first to praise this game for using it in the plot to attempt to reverse Jeralt's dealth.
This reminded me of FFV when Galuf dies and the party tries to use items and spells on him to no avail. This is actually good plot development because it does actually give off the "death is actually death and not just plot armour for the miraculous reveal later" vibe and since this series is built on the fundamental concept of perma-death, not being able to save Jeralt is a good thing.
But holy shit, does it create plot holes that are just stupid when you try to think about them... and they all stem from Byleth's existance.
First off, because Thales appears to save Kronya from dying here, that is stupid to the nth degree because Solon simply just sacrifices her soul a month later. Was that the plan all along? Why not use some other mook? Or did Solon go rogue? This shit is not explained well because even the game understands that Byleth is entirely uninteresting and just wants to get to the meat and potatoes of the story already (the Holy War that was aped from FE4).
Secondly, why kill Jeralt in the first place? I find it hilarious that when we first meet Jeralt, anyone familiar with the series was thinking "well, here's a Greil clone" except that Greil's death actually means something because now Ike and Mist are stuck in a foreign land without their father and a dwindling band of mercs. Jeralt's death literally feels like going through the motions... except there is no logical reason for him to die. Maybe it's because I haven't done the Church route yet but it really feels cheap if you've played FE4 or FE9 (and maybe even FE13).
My only question was "why is it Chapter 9 already and Jeralt is still alive?". Then the cutscene starts and I'm relieved because I'm not wrong about this obvious death.
The whole thing is stupid.
Now, let's dissect the more serious issue about Byleth; the lack of personality that apparently doubles as a personality.
I had someone on YouTube try to honestly convince me that Byleth's lack of a heartbeat is a character flaw.
Byleth has no character flaws (aside from being thick as shit). Byleth also has no character traits. Byleth exists only for the obligatory avatar wank that has plagued this series since FE12.
A character flaw is a deficiency in a person's moral or ethical compasses that create conflict (big or small) with someone else. Byleth is never in conflict with anyone because Byleth doesn't have ANY deficiencies. Byleth follows Edelgard into the abyss and passively encourages her to keep up with her genocide for "peace's sake" despite clearly understanding how goddamn hypocritical it is AND not actually having a personal stake in the matter since plot armour keeps Byleth alive after the death of the Immaculate One regardless of his role or not. Byleth is pointless in Claude's journey due to simply replacing the mirror that Claude would obviously be talking to instead. It's really only with Dimitri does Byleth actually have some reason to exist... and could have been entirely replaced by another character like Felix who could snap Dimitri out of his funk after Rodrigue's death.
And let's be honest, we're here for the three way war which is actually done really nicely. Sure, Edelgard gets a bit shafted by the writing but she still has a complex and deep character that invokes serious contemplation about ethics.
This game could have been better than Geneology in every aspect... if only Byleth didn't exist.
Oh and the designs are also garbage. God, I hate Byleth.