Yeah, I didn't read that much beyond the point I was at before, so I don't know what else happens during the later chapters that are already available. From my current knowledge though, I don't hate him, yet also don't like him as a romantic partner.
He would do extremely well as the funny, somewhat capable and dependable, but goofy older brother, that's also the take that made me comment before. Not just because I hate that it's so obvious she's not going to go for the prince in the end from the fact that that's just what is to be expected, but also because it would make sense. He had no real family around him, aside from his Grandmother (apparently) and she didn't either. That doesn't mean they have to be romantically compartible, because it can also work as a "let's be each other's family" in the familial. Sense, especially since he took her in as her guardian to begin with. He could "adopt" her, or the grandma could.
I'd say it's clearer because the setup is that she's supposed to be interesting to someone else, and he wants to use her. So, obviously, he's going to be the one to fall in love with her in the end, making her useless to his cause. And also obviously, the prince will actually start caring about her, being exactly what ML wanted initially, but now his affection will become a bother.
I wouldn't go by character design, because that really depends on the story, I think. Silver / Whitehaired MLs are also a thing, and I believe that the other guy might just end up being the Crown Prince, so there's that for his standing. I mean, technically, you are right, but in reality, they could have had it all here.

And I can't help thinking: It's obvious that Roussel is going to end up as the male lead, also with their favorite pastimes and their personalities matching, as well as with the manor and all the servants already turning into her forever home. Also, that's just the standards formular. But I really did hope for this to be different. Because why not go with the prince as the male lead and make the marquis into her family? You know, like a brother; the man who woul walk her down the aisle. Her home she can always return to. Because she doesn't have that otherwise.