He's being such an unnecessary nuisance to her despite knowing that she isn't interested in him at all.
The narrative is set up for them to end up together, but I sincerely hope otherwise. There just isn't any chemistry.
I don’t think the narrative is set up like that. It’s more like Koyuki’s character is what initially draws Minato in, and he may fall for her as he gets to know her. Or more like, he’ll develop romantic feelings for once instead of others for him as has happened so often in the past. He was accomodating to these girlfriends, but romantically reactive. This might be the first time he’s romantically active, or depending on his previous motivations, he’s employing the tools of romance to get to know Koyuki better.
Koyuki on the other hand doesn’t quite know how to handle Minato, in particular since he’s subconsciously reminding her of Igarashi, who by all impressions had an ill-communicated romantic interest in Koyuki in the past.
Via shared interest/appreciation of Miki Koyuki realises that Youta is far different from the way she’d put him in a certain category of people and as such opens up to him in a way only Miki was before. It’s not clear, but implied, that this opening my lead to romantic interest, which Koyuki would be badly equipped to handle.
Youta on the gripping hand simply seems good-natured and easy-going, and most importantly, avoided rubbing Koyuki the wrong way.