SPOILER
So the crown prince and Eugene's fiancee, Roselia, are actually in love with each other or at least the crown prince loves her. I forgot. Eugene didn't want to end the engagement because the marriage will give him power since Roselia comes from a powerful family. But when he finally admitted his feelings towards Hari, Eugene canceled his engagement with his fiance and helped the crown prince and Roselia secure their engagement instead.
This kills 2 birds with one stone because now the crown prince owes him, strengthening the relationship between the future king and House Ernst and he gets to marry the love of his life. (≧∀≦)
SPOILER
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It was implied that he had feelings for her even in the past. They were very close that they can be themselves with each other, talk privately and even hold hands. He didn't want to go to her wedding because he felt guilty about the past and didnt feel worthy attending. Also it seemed he didn't want to see her get married too
(/TДT)/

/spoilers/
kinda shipped her with the bright aqua hair guy but guess that's not gonna happen ;(