I would personally put Byleth between mid and high tier. They has pretty good stage control by using his aerial moves, they have pretty good damage, their kill confirm (DTilt UAir) is consistent, and their moves are strong as heck, particularly their down special, which has super armor and can break their opponents shield. They're fairly slow though, and their high damaging moves are reactable, they can get grabbed whenever they get to use their neutral and down special move, so their high damaging moves are best to use sparingly and not be predictable on using them. I've seen both MKLeo and Pink Fresh's Byleth, and both of them have done so well, it shows that Byleth isn't nearly as mid-low tier as some people thought. Byleth won't be top tier by any means, but it's definitely a viable character.