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Fiancée/Wife (Future) EditRuri's relationship with Shu has been neutral at best. Although both can be seen tagging alone whenever their best friends are absent, Ruri shows complete annoyance over Shu's antics and pervertism. As comic relief, Shu would be the target of Ruri's constant abuse each time Shu commits lecherous behavior. But even so, Shu considers Ruri a valuable companion; due to each other's commitment to their friends. Even Ruri herself would occasionally respect what Shu has to say, even finding his words profound.
Overall, both Ruri and Shu consider each other as "glasses buddies" because of their obvious similarities in eyewear. Throughout the anime series, there are only a few instances where the two can be seen in romantic light, such as that time when Ruri was stoically annoyed when she learned that Shu received love chocolates on Valentine's Day. And there were also times in the manga when Ruri would show a defensive stance on Shu; such when she felt very anxious when a girl confessed her feelings to him, or when she beat him up after flirting with a bunch of girls.
Aside from that, Ruri once said that Shu somehow reminds her of her perverted late great-grandfather; whom she both loved and hated. As a tsundere, she would quickly dismiss any conversation about having a relationship with Shu, but even so, her friends admit that the two would turn out as a lovely a couple. Ruri had even dreamt of being married to Shu, much to her shock. She then shows the development of feelings for him, when she learns his future career as a teacher and his true kind nature but can never seem to dissuade the fact of Shu's nonchalant nature of showing his feelings towards other, dismissing his as someone whom she is uncomfortable to be with. After much contemplation however, and Shu's act of rescuing Rosa - a lost young foreign girl Shu and Ruri found - at the very last second, Ruri came to acknowledge that she is, indeed, in love with Shu, albeit considering him problematic.
Eventually, Ruri came clean to Shu about her feelings for him after being enlightened by the struggles of confessing one's love, asking him to at least consider her as a lover after he had moved on from his feelings for Ms. Kyoko. Having learned this, Shu was shown speechless while Ruri was feeling delighted seeing him dumbfounded as he blushed in embarrassment.
In Chapter 229, it is shown that Ruri and Shu are probably living together and in a relationship, and they are preparing to get married in the future by starting to save money.
It was later found out in Chapter 84 that the woman Shu has a crush on was actually his homeroom teacher; Kyoko. Shu both admired and respected her throughout her time in the school. When news reached out that Kyoko was going to be married, Shu was deeply devastated, but even so he still kept his happy-go-lucky facade in front of his friends. Raku was the only one who knew what Shu was feeling. After a heartfelt talk, Shu tells Raku that it is better that he doesn't reveal his feelings for Kyoko, because it wouldn't matter anymore and that Kyoko is now going to live a new life, without him dragging her back. However, Raku manages to see that Shu regrets what he's doing, so he rallies him up to go out and seek Kyoko. Just as Kyoko was already leaving in a taxi, Shu caught up with her and confessed his feelings. She was touched by Shu's actions, and tells him never to let his youth go to waste because she wants him to be happy.
Raku and Shū have been best friends since their childhood having known each longer than the rest of their friends (Chapter 88). Shū had no problem in associating with Raku in spite of him being from a gang family. Despite Raku's occasionally visible unease over Shū's frequent perverted behavior, the duo have an unbreakable bond of trust and friendship that binds them, and sees them open up to each other in their respective times of crises.
Shū has always been there when Raku needs to talk about his feeling towards Chitoge or Kosaki, while Raku stood by him when he was put in the situation of Ms. Kyoko's marriage. Their unbreakable friendship however, doesn't spare Raku from being the victims of Shū's nasty pranks, leading the latter to get clobbered by the likes of a Chitoge and Tsugumi. He also was one of the first to find out that Chitoge and Raku weren't truly together. Shū also reveals secretly to Ruri that he holds Raku in high regard, leading to him always attempting to urge Raku to confess his true feelings to Kosaki on his own accord, instead of setting up a confession; showing his care and loyalty to his best friend.