Now that Dan has been properly introduced, I'd like to take some time to explain why I think he's such a fascinating addition to the character dynamic.
But before I can explain that, I must first explain the other love rival, Kinjo Isamu. Kinjo was introduced the same volume as Niki Mariko, and was clearly meant to serve a similar purpose. Where Niki is the love rival to Kurita, Kinjo would be the love rival to Yamaoka. Niki fulfilled her role expertly, softening up Yamaoka at times, and most critically, getting a rise out of Kurita. Her aggressive forwardness, coupled with her rich and influential family, made her an actual threat to Kurita (even if we know Kurita is the destined one, what's important is that the characters themselves feel challenged).
Now what about Kinjo? Kinjo was able to maybe bring Kurita on a date once or twice, but he ultimately didn't really incite anything in neither Kurita, nor Yamaoka. It feels like Kariya designed him to be the exact opposite of Yamaoka: Physically fit, passionate and dedicated to work, and very social. It just so happens that none of these are traits that really call out to Kurita (I mean, she's in love with Yamaoka, so come on), which means she doesn't have any real romantic chemistry with him. And Yamaoka is dense as a burnt cheesecake so he doesn't feel threatened either. After several volumes in this 'love rectangle', Kinjo comes out looking very ineffective.
And that brings us to Dan. I suspect Kariya must've realized how ineffective Kinjo has been, but couldn't overhaul his character because that would feel too forced. And so, he created Dan, a new love rival for Yamaoka who was designed to be the opposite to Kinjo, in ways that would actually build proper romantic tension. The most obvious one is that Dan is rich and powerful, and while that's not a trait Kurita looks for per say, it is a trait that makes him conventionally desirable in the eyes of many characters, whose judgement does weigh in on Kurita's own. He's also written to have humility and a general good character, which is important because that IS a trait that would appeal to Kurita. And finally, he's proactive, perhaps as aggressive as Niki in love, which means he forces Kurita to make choices and not be wishy-washy. This was all calculated. It has to be. Dan has perfectly answered every shortcoming of Kinjo's character, and his introduction would spice up this love conflict situation quite a bit.
There is one question that hasn't been addressed though, that being Yamaoka himself. Dan does not threaten Yamaoka, because Yamaoka is still ignorant of his own romance. But, he does threaten Kinjo. We'll see in time, how this situation will unfold. Until then, thank you for listening to my TED Talk.