"Only you would be bold enough to bring up wanting to try doing something no one's ever done before, Tsukasa-kun. And that's why you're so wonderful!"
— Rui
Rui and Tsukasa are second year high school students and members of the theater troupe Wonderlands x Showtime. They are a "well-known duo of weirdos" at their school due to the fact they cause ruckus around the campus daily, which most of the time is due to Rui's experiments. According to the "Heat Up! Kamiyama High Cheering Squad!" event, the rest of the student body refers to them as "Oddball One Two."
Tsukasa and Rui first meet during their unit's Main Story. Tsukasa is looking for people to join the newly formed troupe, and he's impressed by the freelance solo performance Rui does at Phoenix Wonderland. He eventually tracks Rui down to their school's roof, where Rui had been waiting while he watched Tsukasa search the school for him via his drones. After a brief conversation about being each other's fans, Tsukasa asks Rui if he wants to join his troupe, which Rui initially denies but later agrees to after Tsukasa assures him that he will perform any role Rui gives him to "12,000% satisfaction."
The troupe is then fully formed after Rui also invites Nene to join. Throughout the story, it is brought up time and time again that they are a dynamic duo when it comes to performance. Tsukasa is always being pushed to his limits by his director because, according to Rui, "he is an actor worthy of the stage" and can always go the extra mile.
The most notable development of their relationship is in chapter 13 of their group's Main Story. On the night of their very first performance - which had failed due to technical difficulties - Tsukasa blows up and blames Nene for ruining the show. Rui, being close friends with Nene, harshly criticizes Tsukasa for this, deducing him to be nothing more than a shallow narcissist who sees the shows they performed together only as a means of gaining popularity. Tsukasa is able to reconcile with Nene, but he is unable to convince Rui to rejoin the troupe even after he rediscovers the real reason for him wanting to perform - because he wants to make people smile. He decides to put on a show for Rui in the group's SEKAI to convince him that their intentions really do align and to apologize for forgetting that, and Rui, touched by this, then rejoins.
In the "It's On! Wonder Halloween!" event, Tsukasa is injured by one of Rui's robots while rehearsing, and, despite the injury ultimately not being very serious, this causes Rui to start holding back in his stage direction because he subconsciously starts to fear that Tsukasa will leave him like his old friends did. The rest of the troupe notice this change, and Rui eventually gets into an argument with Tsukasa about it, which ends in Tsukasa shouting at Rui in frustration. Tsukasa explains to Emu later that he was more angry at himself than at Rui because he "killed the director's confidence." The two are able to reconcile the next day after Rui realizes what his own problem was and both of them come to understand where the other was coming from.
The event highlights the mutual trust, concern and respect they both have for one another through the lens of Rui trusting Tsukasa to perform excellent feats, being concerned for his wellbeing despite this, and respecting his wishes to continue, as well as Tsukasa trusting Rui with directing the show, being concerned for his mental wellbeing despite this, and respecting his personal boundaries. It is brought up again in "Revival my dream" that while Tsukasa does believe Rui's directions are crazy, he also fully trusts that they're safe.
In the current canon, it is generally noted that Rui sees Tsukasa as an actor worthy of a stage, being comfortably confident in giving him often taxing and generally dangerous stage directions with the knowledge he can handle it. This is mirrored in how Tsukasa sees Rui - "awesome but crazy."
In the "Kamiyama High Festival!" event, Rui was Tsukasa's partner while rehearsing the proposal part of Romeo and Juliet for their play. Tsukasa played the part of Romeo, saying "O Juliet! Sun of my life! I beg you, allow me to have your hand in marriage!!" and Rui played the part of Juliet, responding "Romeo... Then refuse your name and I shall swear my love!" Tsukasa closes the scene, saying "Sweet Juliet...!" This scene could be seen in KAITO's Secret Guidance card, where Tsukasa could be seen kneeling on one knee while extending a hand to Rui who is in the corner of the frame. In this card's side story, Rui says that a lot has changed for him since Tsukasa invited him to join Wonderlands x Showtime, and KAITO remarks to himself that Tsukasa has been a positive influence on him.
