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Shu's relationship with Ruri 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. Throughout the anime series, there are only a few instances where the two can be seen in a romantic light, such as the time where Ruri felt stoically annoyed when she learned that Shu received two love chocolates on Valentine's Day. Overall, both Ruri and Shu consider each other as "glasses buddies" because of their obvious similarities in eyewear. Although Shu tends to annoy Ruri from time to time, Shu would also cheer her up in times when she feels down (Chapter 115).
In Chapter 223, Ruri finally admits to Shu that she's in love with him, making Shu shocked for the first time in his life, even though Shu admitted that he still loves Kyoko-sensei.
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 now.
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. Despite Raku's occasionally visible unease over Shū's frequent perverted behaviour, 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 abit Chitoge or Kosaki, and 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 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.