I don't mind MC divorcing the Duke and going with this man but I bet the author just uses this character to help MC get confidence, get the Duke jealous and create some drama. Our dude will just get tossed away or given a consolation prize.
Btw try reading "bakuman" so that you can learn something one or two about manga. Oh try reading Mushoku Tensei novel, maybe you'll something from the mc. The novel, ok? it gives more details and emotions. You'll realize what I mean if you them.