I dont know how or when it happens.
But I usually have this party:
aiko, raphael, then and lucas.
Every npw and then, mostly when in a fight, I realize that the damn Emily has replaced Ethan and is back in the party.
So yeah, not sure how or when, but Ethan sometimes gets switched back to that useless Emily. Think it's cause she and the other 3 were the original party.