I don't think Rui seems different from any other alpha for Tsukasa. Sure, the omega thinks he is tall and handsome when they meet at a ball, and his scent piques Tsukasa's curiosity since he smells kind of different from other alphas, but nothing else. It's what Rui talks about what makes him interesting; sure some of the alphas that try to take Tsukasa's hand in marriage know about literature and theater but Rui has a much deeper understanding about the technical aspects that leaves Tsukasa wondering who this guy is. He has for sure never seen anyone like Rui before. I think their first meetings are short, Rui talks while the dance in one of the many balls organized by Tsukasa's family in order to find the correct candidate for Tsukasa's marriage and always leaves Tsukasa with some kind of question about shows that are being performed in the kingdom. So Tsukasa starts looking forward to these instances where he gets to talk more about his interest while also learning so much, since Rui has a different perspective of things being from the enemy kingdom.

Rui only reveals where he comes from when he brings Tsukasa a book from the black lily's personal library, something Tsukasa had never even heard of. But the white lily won't let him go empty-handed after receiving such a wonderful present! The omega too chooses a book for him, and makes Rui promise to come see him at night again, so they can talk about their ideas and what they thought about them. And so their secret dates start; soon meeting only some nights together to talk isn't enough because they have so many things to say... Saki is shocked when she finds Tsukasa totally-not-trying-to-seek-out-of-the-palace, she swears she sometimes sees a shadow leaving Tsukasa's room when the sun is about to come out in the mornings, but she never says a thing. The only time Rui almost got caught was when Tsukasa felt asleep on his shoulder while they read together in the most remotes of flower fields, where the sea of lilies stops growing. It's hard so enter through Tsukasa's balcony while trying not to be seen by anyone while also carrying the white lilies' prince. Sure Rui needs to leave quick after that, but it's hard when a half-asleep Tsukasa ask him to stay because he misses the alpha when he is gone. Rui needs to find a way to make their dates more regular. Ganba (๑•̀ㅂ•́)و✧