TouKasa

TouKasa is the slash ship between Touya Aoyagi and Tsukasa Tenma from the Project SEKAI fandom.

Canon
Touya and Tsukasa are both students of Kamiyama High School. Tsukasa is a second-year student, while Touya is his junior by one year. The two are also childhood friends who knew each other through their parents' acquaintanceship. Touya highly regards Tsukasa with respect and admiration, while Tsukasa is shown to care for Touya dearly like family.

Tsukasa often points out that Touya smiles a lot more often now compared to the past, and accredits this to Touya's singing partner Akito Shinonome and Vivid BAD SQUAD.

Tsukasa is infamous at their school, known as a troublemaker duo named "Oddball One-Two Finish" with Rui Kamishiro. However, Touya doesn't seem to pay attention or comment on this fact. Akito, who very much wanted to avoid the two seniors at first, gets a surprise when Touya introduces Tsukasa to him as the upperclassman whom he respects.

Touya has a fight with Akito in Vivid BAD SQUAD's Main Story. When Tsukasa finds Touya looking downcast because of this, he brings him to the arcade where they also meet up with his sister Saki. After they play a rhythm game, Saki points out that Tsukasa makes very exaggerated reactions upon his own defeat, to which Touya chuckles unknowingly. At the end of the day, Tsukasa comments that Touya appears to be looking much better than earlier, and tells him that he can rely on him anytime, as he is his kouhai after all.

After Touya and Akito resolved their issue, Touya goes to thank Tsukasa along with Akito. During their conversation, Akito teases the upperclassman, to which Touya would quietly scold him. When Akito casually refers to Tsukasa as "that guy", Touya corrects him and says that "It's not 'that guy', it's Tsukasa-senpai."

KAMIKOU FESTIVAL!
In the event Touya watches Tsukasa's Class Play together with Akito and Mizuki Akiyama. Akito comments that Touya appears to be very excited for it. Throughout the whole play, Mizuki laughs at the absurdness while Akito sighs in exasperation, all the while Touya marvels and continues to praise Tsukasa even by the end of the play, convinced that there should be deep meaning behind the (unintentionally) comedic screenplay done by their upperclassman. Mizuki wonders aloud if Touya's under some sort of spell, and Akito takes this opportunity to ask why Touya adores the upperclassman so much.

Touya explains that Tsukasa is his savior, and begins by saying that they've known each other since childhood thanks to their parents being acquaintances from way back. Touya describes his grueling childhood where he was made to practice the piano and violin every waking moment aside from school. In middle school, he finally grew tired of that lifestyle and wanted to run away from it. When he was hesitating to do so, Tsukasa was the one who gave him encouraging words to push his back, saying "Your life is yours to own, so you should spend it how you'd like. You can simply tell people that you don't like the things you don't like." Thanks to this, Touya was able to quit classical music and pursue street music instead. This is the reason why Touya regards Tsukasa as his savior, and says that if Tsukasa didn't say that to him back then, he wouldn't be the person he is today.

The Tenma Household's Hinamatsuri
The event focuses on Tsukasa and Saki, but Touya (along with Honami Mochizuki) plays a large role in the story to help the siblings make up after their fight.

Quotes
"If not for Tsukasa-senpai’s words back then, I wouldn't be the person I am today. That’s why he's the person who changed me... my savior."

- Touya, from KAMIKOU FESTIVAL! Chapter 4

"In the past, I used to try so hard to make you laugh, but it seems like I don't need to do that now. You're like a little brother to me, and following after Saki, you're my #2 Fan. I should thank Akito for making you smile."

- Tsukasa, from A Bond Between Siblings Card Story 1

"I also have brothers, but we're apart in age. And they studied abroad while I was still young, so I don't have a lot of memories of us bonding as siblings. Besides, back then, I wasn't allowed to play with friends. It was very painful for me as I practiced the piano and violin. But ever since that day, my parents would leave me at your house and we would play together, and you'd show me plays. That had become my emotional support."

- Touya, from A Bond Between Siblings Card Story 1

"...Come to think of it, Touya dragged me along to buy some stuff with him last year around this time too. So that was for Tsukasa-senpai's birthday, huh."

- Akito, from his birthday line for Tsukasa (May 17, 2021)

Fanon
The fandom often groups Touya together with Tsukasa and Saki of the Tenma Family. This is also because of their relationship being described to be like that of siblings in canon.

Although some people prefer to see their relationship as platonic, others also like to see their relationship in a romantic light. The unexpected relationship between the stoic and loud duo, their childhood friend dynamic, and the contrast between Touya's usually cool and poker-faced demeanor with his sparkly-eyed expression when praising Tsukasa are a few of the ship's appealing points.

The ship is less popular compared to the other slash ships involving the four human male characters in the series, AkiToya and RuiKasa. Because of the characters involved, some people who ship this might also ship RuiAki. Sometimes people ship TouKasa and AkiKasa together as Kouhai Sandwich, the poly relationship between Touya, Tsukasa, and Akito.

Fandom
FAN FICTION

PIXIV

Trivia

 * Tsukasa's mother is a piano teacher and Touya's father is a world-famous pianist. While the details of their parents' acquaintanceship wasn't explicitly mentioned, it's easy to conjecture.
 * Tsukasa and Touya share the same birth month (May) with a gap of 8 days between their birthdates. (The 17th and 25th, respectively. Saki's birthday on the 9th of the same month also adds to the 'Tenma Family' description.)
 * In the game's default party names for Cheerful Carnival events, Touya and Tsukasa in the same team together would have the optional party name of "Respect and Deep Affection".
 * In the 24th Project Sekai Summer Vacation Radio broadcast for Wonderlands x Showtime, when asked which character they would want as a brother, Tsukasa's voice actor Daisuke Hirose answered "Touya".