
It wasnt because he had a thing for her, its his hatred for the Emperor's family! To screw them over he wants his own child on the throne.
reasons below.
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He has a grudge against the Emperor's family as his mother was one of the Emperors fathers (EF) comcubines who loved the EF and was loved by him, she helped him gain power and when that was done he forced her to marry a king in another continent.. Breaking her in the process.
This is revenge for his mother who had eveything stolen from her by the emperos mother and father. So to screw over their child he chose to help Heinley, while also using Trasha.
He told her to become empress because he sees her greed and knows she isnt fit. He knows the Empress is a perfect Empress and makes up for her husbands "mistakes" so without her the empire is screwed.
He slept with Trash to get her closer to the Emperor knowing its his own child the Emperor will walk through lava to put on the throne.

and i saw in one of the comments at some site that Duke Elgy's daughter will be a knight for Navier's daughter who will one day rule for the west empire. And the Navier's son will rule the eastern empire.
but please do tell me if i'm wrong. i just saw that comment in one of the comments at some site haha
a lot of people are getting tired of seeing trashta and sovieshu. I'm just here enjoying how that guy who told trashta about becoming an empress. Like sure become the empress. The guy is actually on Navier's and Heinley's side, I think. He is purposely trying to push the idea forward so things move faster for Navier to be with Heinley! (omg! I love it!) Like. Duke Elgy is actually us! He wants things to go faster so we can see Heinley and Navier live happily ever after and see Sovieshu ruin all his chances and future! Keep it up dude!