Byleth, Dimitri and I are from three different Fódlans, where events diverged pretty dramatically. As Dimitri would have it, Byleth picked the Blue Lion house as its teacher. The way I remember it, of course he couldn't say no to teaching his adorable little Golden Deer. --That was my house, back in the day.

[Which probably explains all his oblique animal metaphors he likes to make sometimes... but anyway.]

And for the Byleth we know here? He never became a teacher, but eventually joined Dimitri's army as a sellsword. I was basically a stranger to him.

[Which was tough, at least at first, but he's long since come to terms with it.]