Here's a TRVKE for ya
Fact is, Aether is the promotional necessity. If Lumine was the poster protag the game would be perceived as for girls. The majority still overwhelmingly pick Lumine as player character when given the choice. When you install genshin you are already sold and Lumine is another cute girl, a product, that the player is given. The story also necessitates that one will be in opposition, so both can't be promotional material as the player choice.l, contrasting granblue where both protags walk the same path with minor deviations.