so who

It's obviously the black haired guy, i know these tropes so well, she'd be partnered with the enemy's son

Black haired guy is not enemy's son tho

yeah his dad is not related to the queen but I can understand why u thought that. they do both have the same hair color but the Duke has blue eyes and not amber like the queen's brother and herself

The duke is related to the queen, they were family, the queen called the duke "brother". Even if they weren't blood related, they were in the same faction, opposing the princess to take the throne.

No, the queen's brother is a different person, they just look similar (⌒▽⌒)

Lol the queen's brother look older and have goatee, but the other duke's face is clean

Well, my bad. They definitely weren't the same person but still i believe the duke is on the same faction as the queen. Anyways the ml is always the darker kid who have some issues, and the easy outgoing guy is always the second lead or a candidate, it's a trope u'd always see on shoujo or k-drama.