RuiKasa

"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 in "Mascot Greeting" first side story

RuiKasa is the slash ship between Rui Kamishiro and Tsukasa Tenma from the Project SEKAI fandom.

Canon
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, before he runs away from the guards for performing without permission. 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.

In chapter 2 of Tsukasa's 3* "An Officer's Crisis" card story from the "Revival my dream" event, Tsukasa remembers what happened during the Halloween show and starts to worry that Rui will hold back again in their current performance, as it also involves a risky stunt that had almost injured Emu earlier in the story. This leads to him staring at Rui out of concern as the group take a break from their rehearsal until Rui notices and asks him if there's something wrong. Nene eventually points out why he's worried, and Rui reassures him that he's okay this time, saying it's all thanks to Tsukasa for going all out with his directions back during the initial issue.

In the "On the Stage of Dazzling Light" event, Rui worries about not being a good director for Tsukasa as he feels he held him back by not giving him enough different roles in their shows, and he spends most of the event looking for ways to help Tsukasa with their upcoming performance. His 4* card story also showcases this, as much of chapter 1 is him talking about Tsukasa and asking Miku if she can help him out. The two of them also have a duet section in the event's song, "88☆彡," and extra illustrations in the song's MV.

In Tsukasa's 2021 birthday lines, it is stated that Rui made him a celebratory cannon filled with confetti, the dialogue being: "Tsukasa-kun, look! All of the confetti flew up so high. Now, if they manage to spread out beautifully, this will be a huge success!" To which, Tsukasa's response was "Ooh! The celebratory cannon made by Rui is amazing! It looks like as the whole school is covered in a flurry of confetti!" This is also confirmed by Nene and Emu, both mentioning the huge cannon.

In the 2022 Gentleman Thief's Thrilling White Day event virtual show, Rui mentions wanting "to make a chocolate statue of Tsukasa-kun and then having the real Tsukasa-kun appear from inside of it". When Tsukasa asks how he'd do this, Rui whispers something to him, and Tsukasa then asks how that'd even be possible.

In Rui's 2022 birthday card lines, Mizuki remarks that they've noticed Rui's changed a lot for the better since they knew him, and wonders if it's because of Tsukasa's influence. Tsukasa also tells Rui not to hold back with shows or even in private, and to tell everyone if there's anything he wants to do with them.

Fanon
RuiKasa is widely considered to be the most well-known and most shipped slash ship in the fandom. The ship is supported mainly due to the pair's natural chemistry, and also the fact that they're both very popular characters. It is the most written ship on AO3 with over 2000 works; even managing to get into the Top 100 ships in AO3 of 2022. They are also the ship with most works on Pixiv as well. It's mainly rivalled by Ruinene and Emukasa, as well as Ruiemu and Nenekasa, but it's also common for all five ships to be shipped within Polyshow. RuiKasa also has slash ship rivals as well, with some also shipping all four of the game's male characters together as the poly ship Fantasista Squad. RuiKasa is often shipped alongside Emunene.

Tsukasa and Rui are often perceived as a sun and moon duo, with Tsukasa being the sun and Rui being the moon. This is due to Tsukasa and his cards being lighter and brighter in color scheme while Rui and his cards are darker in color scheme. People have also pointed out that the moon appears in Rui's fragment SEKAI while Tsukasa strives to be a star, which is what the sun is. Nene also walks away from Rui and Tsukasa in the Kami no Manimani 3DMV while she sings the line "And sleep without regard for the sun or moon." After the "Towards the Phoenix at the Sky's Edge" event, fans have said that Rui and Tsukasa are swapping places, with Rui becoming the sun and Tsukasa becoming the moon, but the majority of people still view Tsukasa as the sun and Rui as the moon.

A RuiKasa Ship Week was held from December 25-31, 2022. Prompts for the first day were "holiday, confession or hanahaki", "angst, dressing up or unit swap" for the second day, "future, AU or piano" for the third day, "first meetings, sleepover, cooking or baking" for the fourth day, "sick day, magic or personality swap" for the fifth day, "practice kiss, first date or classic plays" for the sixth day, and seventh and final day was free choice.

The second RuiKasa Ship Week was held from July 12-18, 2023 under the. The prompts were "summer festival, mythology or soulmates" for the first day, "beach, stargazing or AU" for the second day, "amusement parks, song or horror" for the third day, "bad weather, ocean or music" for the fourth day, "vacation, hospital or detective" for the fifth day, "flowers, angst or cats" for the sixth day, and seventh and final day was free choice.

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Trivia

 * Their ship is referred to with the "🎈🌟" and "💜💛" emojis.
 * Their Kizuna Rank titles include: "Weirdo Combo", "Top☆Star?!", and "The Best Show".
 * They have a height difference of 8 cm, Tsukasa being 172 cm and Rui being 180 cm tall.
 * They currently have covered 5 songs together (,, , and ), two of which being without an accompanying VIRTUAL SINGER. They also both have solo versions of Rui's second commissioned song,.
 * Nonsense Bungaku was given to them as a duet (feat. Len) during the Eve tie-in. It is about someone who was lonely but starts trusting again after they meet a certain person, which is reflective of Rui's development with regards to Tsukasa throughout the game's story.
 * In the KING 2DMV on Project SEKAI's official YouTube channel, they are shown as playing cards with Tsukasa as the king and Rui as the joker.
 * In the 3D performance of the same song during Wonderlands x Showtime 1st Connect Live STORIES, their choreography during the pre-chorus featured both of them reaching out to each other and Rui tilting his head backwards when Tsukasa's hand was below his jaw.
 * In the Sweet Magic 3DMV, Tsukasa runs over to Rui and gives him a fistbump with a wink and smile.
 * The song commissioned for Wonderlands x Showtime's Main Story,, mentions a "thick-skinned king" and an "alchemist full of desires", referring to Tsukasa and Rui respectively.
 * In Nene's initial 3* second card episode, she tells Emu that Rui and Tsukasa had to stay back at school for detention because Tsukasa almost got blown up in the school's swimming pool.
 * Their image and hair colors, yellow and purple, are complimentary colors. Their secondary hair colors, orange and blue, are also complimentary.
 * As of the "Connecting Painful Hope" event, Tsukasa is the character Rui has mentioned the most in his stories, at a total of 329 times. This is over 100 times more than Nene, who is second at 197.
 * In their 2022 birthday card art, Rui wears a white cravat while Tsukasa wears a yellow necktie. In their unit outfits, Tsukasa wears a white cravat while Rui wears a yellow necktie.
 * Additionally, the flower Rui is holding in his card is an aster, which means "star," and Tsukasa is associated with the stars.
 * Tsukasa and Rui are often respectively associated with the numbers 1 and 2 both in the game and in promotional material.
 * Their first Kizuna title is "Henjin 1 2" in Japanese.
 * Tsukasa was one of the limited cards for the game's first anniversary event, and Rui was in the second.
 * Their art for the summer memory photos was posted for day 12 on the 21st.
 * Additionally, Tsukasa's promotional art for the game's second anniversary was posted on day 21, however Rui's was posted on day 13 rather than 12.
 * In their unit's cover of, they are the ones to say the "one, two" lines at the beginning of the song.
 * Some fans like to believe that there is a correlation between certain lyrics in and, which are Tsukasa's second focus song and Rui's third focus song respectively. Tsukasa sings the "I wish happiness will come to that kind of you" line in his song while Rui sings "I know the ending will be a happy one" at the very end of his song. (However, another common theory is that Tsukasa's line is directed towards Saki instead.)
 * In 's 3DMV, Rui sings the line "Loneliness melts away with the touch of passion" while he waves to Tsukasa.
 * In Rui's 4 star card, "Appreciate That Spirit of Yours" from the On the Stage of Dazzling Light event (Tsukasa's second focus event), the book he holds has stars on it that very closely resemble those in the Nonsense Bungaku 2DMV.
 * Rui and Tsukasa's Mastery Rank 5 outfit variants from the On the Stage of Dazzling Light event share the same/similar colours.
 * Additionally, their "Sorrow Farewell At the Curtain Call" (Rui's third focus event) outfit variants from Mastery Rank 1 reflect each others default outfits from that event; Tsukasa's MR1 variant shares the same colours as Rui's normal "Hourglass Tuxedo" outfit, and vice versa for Tsukasa's "Advanced Tuxedo" outfit.