Do not throw rocks at me but Byleth as an antagonist is much more interesting than as the main character.
Like think about it. A woman from a basically extinct species of ancient dragons obssessed with having her mother back, for years and years conducting experiments and creating test subjects to adquire the power and HEART from her mother and turn them into her, all the while being the head of religion of the land and dictating what is sacrilegious and what isnāt, even hindering technological progress, and at the same time fighting against the people that wiped her people in the first place by subjugating all under her order, until eventually she succeeds- she creates the perfect vessel. The perfect equilibruim of strength and speed, with all of her motherās powers, and at the cost of overwriting them as a person. A perfect killing machine that will listen to her and her only.
Thatās fucking terrifying. Itās an amazing setup and a great way to show how far Rhea has gotten, and makes her a great villain, showcasing both her ideal, her drive, and her ambition in a grey manner. Byleth being an emotionless villain makes us see how far gone Rhea is without woobifying her the way 3 Houses did bc from Bylethās point of view, in 3 out of 4 routes they saw her as someone who cared for them, and appreciated them and their work, even if she had at the start different plans. 3 Hopes is from a weird ass kidās point of view, and from their point of view, we can clearly see how ruthless Rhea is to bring her mother back to life. Now Byleth being silent and showing no personality isnāt unbereably boring and dull as a character. Them being silent and having no personality is a nice example of show donāt tell that the dead should be allowed to rest in peace, lest be at the cost of someone elseās individuality.