This.
Seriously, what the heck is up with that? Even if Jeralt is avoiding the church, it's damn near impossible to raise a merc that never even heard of it.
Also, the whole famous merc makes the idea of him blocking an axe with his spine even worse. Like bro, if these are your battlefield reflexes, you got hard-carried by Jeralt.
Byleth is a whole mess that I just can't take seriously. They fail as a protag because they're all but a cardboard cutout. It would make sense as an avatar, but they also fail on that front because they're as customizable as a cardboard cutout.
Seriously, it doesn't take a lot to get an avatar going. Just have more customization options and especially some dialogue options. They already have branching dialogue in 3H, but it basically amounts to nothing because all the options are samey.
I'm not buying the pre-rendered cutscenes argument either. The reason is very simple. Why do they even exist to begin with? In-Engine cutscenes have been the standard in gaming for a minute now and IS used them on the 3DS.
Somehow, even in the context of 3H, Byleth manages to be a bad avatar gameplay-wise. As you know, Byleth starts with 13 Strength, 45% Str growth, and gets a 1-2 Sword with a solid combat art. In contrast, if you want to go down the magic route, they start with 6 Mag, 35% growth, and a dismal spell list. Great job.
Also, everyone has access to all classes depending on your actions barring Gender-Locks and the like. All they need is the proper ranks. Speaking of which, the good old professor does NOT do well on that front.
Since Byleth is the teacher, they can't attend classes. And boy does that make getting said ranks with them an enormous pain.
Spending activity points on Byleth's training instead of meals and the like means you're going to lose out on 2 students' training sessions unless you bribe them with gifts and lost items, and you're missing out on professor level. The closer to part 2, the less of a problem this is, but the early-mid game is crucial because of how important the Intermediate classes mastery skills are.
They don't even have Bow proficiency. Their whole skill set is based around being adjacent to allies so they can get more exp, do more damage and all that stuff, but when it counts, all Byleth has to offer is a solid bonk with the Sword of the Creator. (Which defeats the purpose as that's more of a killing tool than a weakening tool.)
It's not like they're bad as a unit, they are pretty good, but man, that's not an avatar kit at all!