The gradual buildup of Nariyuki x Uruka

I'm not saying these things because I'm rude, but seriously, you have to look beyond the usual MC blushing to every heroine routine.

Rizu kissed Nariyuki early on, and further wanted to experience. Outside of Nariyuki telling her that what she is doing is not normal as they are not a couple, nothing came out of this. He was flustered around Rizu for fanservice reasons, while Rizu continued to understand emotions more and more. This was Rizu's character arc. Nariyuki not even once considered that they should be a couple.

Fumino had it worse. I'm willing to accept that she had good chemistry with Nariyuki, but her denial about her own feelings and her support lasted way too long. Then both recalled their memories together and Nariyuki felt nothing for her. She Onee-san-zoned herself, love counselor-zoned herself. Her character arc was about solving the issue with her father.

Mafuyu and Asumi are both awesome, have good chemistry with Nariyuki, but they never really wanted to go for the bowl that seriously. Nariyuki likes them for what they are, but nothing more. Mafuyu was a good mentor/foil to him, thanks to meeting him, Mafuyu mended the relationship with her students and parents. Despite being needed to take care of, she helped him time to time with giving him a car ride and such. Asumi never wanted them to become a real couple, neither Nariyuki insisted anything like that even though she would have a good shot. But she never gave up following her dream and become a needed, friendly doctor.

Rizu, Fumino, Mafuyu and Asumi had personal character arcs. Nariyuki was involved, but the end result was never that the girl and Nariyuki were mutually developed together in the arc. Nariyuki being the one and only existed from their POV, and not the opposite, where Nariyuki gets something more other than the blushing routine.

Being mutual was Uruka's and Nariyuki's thing. Most of their key points:
Chapter 7 highlights studying. Nariyuki didn't like studying initially, but as he watched Uruka swimming and didn't have talent for something else, he learned to focus on things and putting in effort. Uruka thanked him for the lesson and highlighted that she still hates studying, but it's more fun with friends and she is grateful for Nariyuki for that. Nariyuki noted that maybe tutoring them is actually not that bad.
The recurring plot point of Uruka's non-existent boyfriend. Nariyuki always asked for details and confirmation about the guy, as he actually wanted to hear there is nothing between them. He directly picks a bra for Uruka to wear in Chapter 28, his biased choice, and he is absolutely delighted when he saw Uruka wearing it in Chapter 33.
The recurring plot point of awkward romantic tension between them. Fumino mediates in Chapter 35 and 83 as well. Only they get these types of chapters. During the first instance, Nariyuki compliments Uruka's hair. The next day no matter how Fumino flaunts her hair, he says nothing. Not just that time, never actually when she changes her hairstyle. On the second instance, Uruka rather not tell Nariyuki the fake boyfriend thing and that she likes him, because she would be obstructing Nariyuki career as he always occupies himself with more than what he can handle. Nariyuki actually feels the same, as long as it's good for Uruka, he is fine with her leaving, but she will be lonely. He gives a definite "OF COURSE I DO CARE ABOUT URUKA" to Fumino's teasing, which engraved in Fumino's mind. It was clearly more that what meets the eye. And of course the inbetween Chapter 53, with the kiss on the bus, the chapter where they witness Umihara and Kobayashi smooching and they feel awkaward again next to each other, Nariyuki thoroughly observing Uruka's beautiful lips, wondering if he dreamed or not about her kissing her, and what even caused such thoughts of him wanting to experience something like that.
Chapter 42-43. Nariyuki ditches cram school on his own just to see Uruka's swimming race. He got mesmerized by her beauty, noting how she is brimming with light and how she is especially beautiful today. But above all else, Uruka told him that of course she is frustrated over losing, but before they can cry their eyes out, they must do their best. Nariyuki was moved. They sneaked in their middle school to cheer her up. Uruka would have quit the swimming team if wasn't for Nariyuki insisting she should stick. Uruka thanked her. Nariyuki felt the same. When his dad passed away, Uruka's swimming achievements gave him the strength and inspiration to overcome his struggles. The Uruka who was uncomfortable with handholding then grabbed Nariyuki's hand, not because of thirst, but empathy. She felt bad for Nariyuki needing to bring up a sad story like this. Who suggested then to practice a confession on him? Nariyuki. Things got awkward again, but Uruka managed to get away with him permanently calling her on her given name. As far as I remember, she is the only one permanently, the others just occassionally.
Chapter 69. Uruka wears the costume Nariyuki made just for his effort not go to waste. The bystandars noted that the awkwad couple act they put on to sell udon suddenly feels really genuine. And of course she is the default fireworks girl.
Chapter 77, the campus visit. Uruka try to put up distance between them as she eventually goes abroad, but they still end up as a baka couple to the bystanders and Nariyuki actually wanted her to show more skin. Who imagines them gracefully as an university couple, relaxing happily? Nariyuki.
Chapter 90 and 95. Nariyuki almost drowns in the water, but Uruka saves him. She genuinely cries because he means the world to her. Later she kisses him on the lips and announces she goes abroad. Nariyuki has to cope with the situation. On the day of the marathon, she tells this to Rizu. The kiss made him happy and also the fact Uruka is doing great to achieve her dream. But he himself feels he was left behind. He saw Uruka's back running and kicked himself into gear, because he wants to stand as Uruka's and the girls' equal. Uruka gets the important highlight again, as he catches up to her and they are running side by side, while Rizu and Fumino witness Uruka's influence on Nariyuki again.
Chapter 105 is another chapter, probably THE standout Uruka chapter for me, that captured her influence on Nariyuki. She goes straight to the more serious talk that involves Nariyuki's future. "You want to be an educator like your father, right? Looking at your face back then it made me realize that's what was going on." And then intertwining her fingers with Nariyuki's she tells him: "I love the Nariyuki who cherishes his family. But for you to be happy, it's okay to rely on others. There are so many people out there ready to help you after all." basically telling him to be a bit selfish, rely on others and go after what he actually wants to be in the future. This chapter was significant as it gave Nariyuki the push he needed to pursue for his own dream. Unfortunately, the chapter is often overlooked because simple fans were clouded by Mafuyu-sensei getting out of Nariyuki's bath with just one piece of towel at the end of it.
Chapter 128. Again with the side by side, mutual support key aspect and Uruka highlight. You could argue that Uruka didn't take an exam, so she was the definite candidate to find Nariyuki, but if Tsutsui didn't want to make a point here again, he could have done something else. It's not a coincidence they ended up getting a couple cover for Volume 15, instead of the solo covers of the other heroines for their character arcs.
Chapter 132. Nariyuki once again thinks how lonely he will be when Uruka goes abroad as the day of departure gets closer and closer. None of the other heroines got something like this from him.
Chapter 137-138. Valentines. Just like with Nisekoi, where the obvious contenders, Chitoge and Onodera got the spotlight, Valentine's features Uruka prominently. All the other heroines only got a throaway scene, giving their chocolates to Nariyuki casually, without anything building up to an emotional scene or cliffhanger.
Closure - The Mermaid arc. Uruka confesses for real, kills of any misunderstanding between them. Time for Nariyuki to thorougly examine their relationship. As established before, they hardly talked in middle school, so people should just forget their headcanon that Nariyuki fell for her there. Nariyuki tries to pinpoint the exact time he fell for Uruka, but he can't. He tries to answer the question what is Uruka's existence to him. She is his personal sun, he loves her. That's his conclusion. Not because of a specific moment in the past, it's the culmination of their journey, her influence on him both in middle school and high school, highlighted a lot of memories corresponding to my points in Chapter 149. Fumino and Rizu heard his wish to see Uruka, they gracefully stepped down and helped him back, as I also mentioned their character arcs were not about mutual feelings.

So now that I took my time and collected my thoughts, is there still doubt that Uruka didn't stand out, it was not an obvious, conscious buildup from Tsutsui? Back to my first sentence, I don't think I'm rude here. If anyone can list things for the others that could rival these points for Uruka, be my guest. But I don't want to see "Nariyuki made a generic blush there for this girl", "Nariyuki said a generic supportive line there for this girl", "Nariyuki's chemistry with this girl was over the roof" etc. I want trump cards, multiple moments where it was obvious both party are into each other without any doubt and their feelings are mutual or have a high chance to be mutual. And when it comes to trump cards, Uruka overwhelmingly stands out.