[Spoilers] Why Zenos is the most important character moving forward

[RopeDrink]

Let me put it a different way:

Zenos --- Grynwaht

Leader --- Underling
Apathetic --- Enthusiastic
Calculated --- Reckless
Emotionless --- Emotive
Self Serving --- Serves Others
Seeks Challenge --- Seeks Vengeance
Elated by Defeat --- Infuriated by it
Intelligent and Imposing --- Dumb, Brutish Thug

The list goes on.

Even in the scene where Zenos and Grynwaht are in the same room, they are shown as total opposites - Zenos callously and emotionlessly tormenting Yotsyu with his cruelty (to display tension) while Grynwaht is constantly emotive about it, before bursting into enthusiasm at the mere tease of a chance to get his vengeance in a completely comical fashion (to lighten the scene). They don't even fight the WoL for the same reasons, let alone act in any way remotely similar. The list goes on and on to a point where one could see them as total opposites. The only unifying factors are that they a) serve the empire (to some degree) and b) are antagonists, but if the one thing required to make this not matter is "their arc involves the WoL", then this conversation will go nowhere other than disagreement-circles, because that is pretty much the fate of 99% of antagonists anyway. It is cutting corners to compress something into a pulp so they look the same to further an argument.

You can compare their goals - at least then you have some modicum of similarity (even if they are very, very far apart from one another) - but let's not pretend like they are the same character or even similar.