WARNING: This section contains heavy spoilers for Danganronpa V3.
Shuichi seems to strongly believe in Kaede, reassuring her and telling her that "if it's her, she can do it". Shuichi's presence appears to comfort Kaede, due to his status as a detective, although she admits that he seems unreliable. She attempts to help raise Shuichi's confidence by giving him pep talks.
At first, Shuichi admitted to not completely trusting Kaede when murders occur, which disappoints her, although she does accept this since she believes he can solve the case. However, this changed drastically after they began investigating the school together, as he came to admire her for her determination and put much trust in her. He offered to help Kaede numerous times in Chapter 1, even bowing to Miu at one point, merely because he saw Kaede doing so. They also shared a rather intimate moment in a classroom, in which Kaede held Shuichi's hand.
Kaede has been shown to care deeply for Shuichi, as shown in the first class trial, when she attempted to steer suspicion away from him after the other students begin to accuse him. This ultimately resulted in Kaede being voted as the culprit, something which devastated Shuichi.
The two shared a tearful and emotional moment, and Kaede was driven to tears while Shuichi angrily asked her why she had murdered Rantarou, although he stopped immediately when he was reminded that Kaede would be executed. When Kaede apologized to their fellow students, her apology seemed to be mainly at Shuichi. It seems that Shuichi forgave Kaede and tried to rescue her in the beginning of her execution. In the beginning of the execution, Shuichi was shown trying to reach Kaede's hand but his attempt failed and she is now remained dead.
Although hurt by Kaede's supposed betrayal, Shuichi still cried over her demise. He imagined her playing the piano after the trial, and was suddenly motivated to keep moving and break out of the school for her. He still thinks of her from time to time, such as in Chapter 3, when Korekiyo asks Shuichi if he wants to try speaking to the dead. Shuichi is interested in the idea for a second, thinking that there is only "one girl" that he wants to talk to once again. It's very likely that he was referring to Kaede; however, he quickly disregards the idea because he thinks speaking to the dead is impossible.
Because the two spent so much time together, many other students pointed this out and some more or less jokingly assumed they were dating, with Miu even crudely assuming that they're having a sexual relationship. Kokichi also cheekily asked Kaede about how far she's gone with Shuichi, but then claims that he simply meant how far they've gone investigating the building.
During the love hotel fantasy event, Shuichi had the role of Kaede's boyfriend and they celebrated their first anniversary. Kaede seemed upset because he didn't call her by her first name and tried to make him call her 'Kaede' instead of 'Akamatsu-san'. However, after Shuichi tried to call her 'Kaede' for the first time, they both find it hard to regain their composure. After that, Kaede confessed to Shuichi, which caused the latter to become too shocked to say anything. Out of blue, Kaede hugged Shuichi and they fell down on the bed. Shuichi began to panic, but Kaede asked him to hug her even tighter. Kaede admits that she likes hearing Shuichi's heartbeat and that if Shuichi encourages her, she can do anything. Finally, Kaede asks if Shuichi will "do what she wants to do with him right now" and he pulls her closer to him. It's heavily implied that Kaede and Shuichi had sex afterwards.