Thing is, the whole problem doesn't even lie entirely on Rui一MC is also partially responsible.
MC is at the time on her life where love and romance is probably the last thing in her mind. No matter which ghouls expressed romantic affections for her, there's a very high probability that she'd still reject them (regardless of her own feelings towards the ghoul). She wants to focus on trying to break her curse first and foremost.
But the difference between Rui and... let's say for example: Jin and Lyca一is that Jin and Lyca aren't the kind of people who would put on a mask to hide their own feelings. Or even if they do try to hide them, they're not very good at it.
※ Using Jin and Lyca as comparison specifically because there are instances where Rui were hinted to be envious of those two's relationship with MC.
Jin is emotionally constipated. He's distant and tries to keep everyone at an arm's length. He also say things一cruel things一that can be misunderstood and hurt the people he cares about. But as if to make up for that shortcomings, his affection bleeds through all of his actions for them.
He may hide his feelings with cruel and harsh words but his actions speak louder than his words. He wouldn't go through all that trouble if he doesn't actually gives a rat's ass about MC. Would help if he's a bit more kinder about it tho... ┐(´д｀)┌ C'mon, Jin... *pokes him repeatedly*
Lyca is even more amazing on this part. He's very earnest and honest with his own feelings一always wearing his heart on his sleeves even when the world punishes him for it.
Lyca's kind and compassionate dispositions are the things that touch MC's heart and made her often worries about him. He never lie about what he truly thinks and speaks his mind freely whenever he has the chance. This is one of the reasons why MC feels comfortable around him.
And that is exactly what Rui lacks: the fortitude of being honest with one's own heart一to lay his feelings bare and show it through actions.
This isn't to say Rui isn't sincere or that all the thing he's done and said are fake and not from the heart, no. In fact it's the opposite. Rui is a very kind and empathetic person. Maybe even a little too kind. Which is a very cruel irony since the narrative seems to hint that all the unfortunate things happening in his life stemmed from his own gentle, selfless heart.
