RyuKasa

RyuKasa is the slash ship between Ryusui Nanami and Tsukasa Shishio from the Dr. Stone fandom.

Canon
After the end of the Stone Wars and Ishigami Senku being unable to heal Shishio Tsukasa with the level of medicine available, the latter is induced to cold sleep by the former as last resort to prevent Tsukasa's definitive death. While the previous Tsukasa Empire and the Kingdom of Science merges into one party, Senku establishes the search for the source of the petrification phenomena as the main cast's next goal, promising to use the petri-beam on Tsukasa in hopes to get his body recovered from the fatal injuries. Since the very clue about the petrification source is the vague idea that it started on South America, Senku designs a ship and asks Hokutozai Minami's intel about skilled sailors of Modern-timers. Minami's first thought is Nanami Ryusui, but quickly desmises the idea due to her impression of him as a greedy brat. She admits that is brilliant about anything related to navigation and shipcrafting, but his personality was the very kind that Tsukasa would hate the most. Considering their urgence for skills with barely resources at hand, Senku didn't blink before depetrify Ryusui.

After acquiring platinum for the depetrifying recipe and one Medusa device, the Kingdom of Science learned that the mysterious Whyman is located on the moon. This information leads Senku and the others to agree on building a space rocket to land on the natural satellite, needing for such a small crew with: a) one pilot - that Ryusui claims as his position, b) one scientist - that's probably Senku, c) and one powerful warrior for whatever dangers the crew may find on the moon - that Senku states to be Tsukasa's fitting role. After Tsukasa being successfuly revived in standard health, he shows hints of awkwardness to be welcomed to the Kingdom of Science (as also some KoS members displayed a bit of distrust or fear of him), but thanks to Gen's words, Tsukasa put on war paint to imitate his first-petrification marks like everyone else. With Tsukasa joining, the crew is preparing for world-tour sailing, intending to found cities on different continents and get manpower and resources for the rocket project.

About reviving much more people, Ukyo and Yo question about reviving ill-intentioned folks, to what Ryusui states that law and order will be established on those locations and Tsukasa gives an ominous glare affirming that as long as he's there, "It. Will. Be. Fine" - to what others quickly agree with. Following the dicussion, Ryusui challenges the crewmembers to whoever spots land first to win a money prize (100.000 dragos), capturing Tsukasa's attention about the currency at hand and exchanging serious glares with Ryusui. Minami cuts the conversation by lying blatantly about the currency nomenclature and takes Ryusui away from Tsukasa. Later, Minami lectures Ryusui, mentioning how their ideals are opposites and it would for everyone's well-being to avoid friction (while admiting that Tsukasa would hate him, even if Minami changed her first impression on Ryusui by now). Ryusui smiles tenderly and replies that, even if Minami hated him, he loves her and all the women on the world, as well the men - and it would, by his words, include Tsukasa, since Ryusui desires him too. Minami's and Gen's speecheless shock apart, the former reporter counters back that Ryusui cannot control people with money, to what Ryusui comments that money shouldn't control people, but help them to focus on a task and get together to reach a higher potential. Cut to Tsukasa just outside Ryusui's place, listening and smiling to himself.

When the ship crew is getting interviewed by Minami on the departure day, Tsukasa is spotted by Ryusui's side on the captain's cabin. Tsukasa is recorded affirming that his new role was to prevent people trying to take advantage of others on the Science run, hinting that he left to the past his ideal of a society of pure-young-people only. As he got along with the people depetrified after his cold sleep (François, Matsukaze...), Tsukasa is seen also partaking on Ryusui's shenanigans with Senku and others, like the Perseus Luck Casino bets and the Viking skit, setting the peace (or, at least, truce) between the King Lion and the Sea Dragon. As very charismatic and commanding personalities, both Tsukasa and Ryusui settle as dear leaders for different specialities (fighting fronts and logistics), as so they would be seen usually apart from each other given their needed skills in different sections.

Fanon
Months before Tsukasa got revived and had a chance to interact with Ryusui, fans of the manga were joking about the clashing between Ryusui's and Tsukasa's core societal beliefs, what was in-universe pointed out by Minami as one of the reasons to not revive Ryusui at first. Jokes about the capitalist x anarchist dynamics and tension that would occur eventualy fed an alluring idea of how opposites may attract themselves together. It helps that both characters stand between some of the most popular ones from the franchise, competing with their high levels of charisma, überskills, beauty and badassery displayed often along the story. When Tsukasa was fully healed by Senku's petrification plan and met Ryusui, and immediatly their tension was waved out by Ryusui's passionate speech about humankind's union and potential (with not so subtle sexuality implications), the perspective of their relationship changed drastically for the better. Despite the lack of other meaningful interactions in canon after chapter 142, the appeal of their complementary personalities/opposite approachings in addition to what settled as mutual understanding to a degree and status as allies without apparent unsolved matters keeps fueling the small and, arguably, very recent shipping fanbase.

Ship Name Variation
The slash ship consists of Nanami Ryusui, presumably the once heir of the Nanami Shipping Conglomerate before the world-wide petrification, and Shishio Tsukasa, once know as the strongest High School primate in the old scientific civilization.

Both characters are from Dr. Stone series. Variations of the ship name in Eastern fandom includes 司龍 [shiryū] and 龍司 [ryūshi]. Variations of the ship name in Western fandom include RyuKasa, TsukaSui, RyuShio, TsukaRyu, and other variations resulting from Portmanteau Couple Naming. The Western shippers may tag their works with the first two ship names more oftenly, and Eastern creators may simple tag the characters' names, since there isn't a consensus on either ship tag among the series' fans.

Popularity
This ship has a loyal but very small fanbase, with 60 fanfics featured in AO3, notably gaining more attention between manga readers since North America Arc. Often, this ship works as Combo Ship with SenGen (Ishigami Senku/Asagiri Gen). On Tumblr and Instagram, tagged arts are less than 50 in each social media, and on Pixiv the tags accumulate less than 100 entries between fan arts and fanfics.

First seen in: Manga Chapter Z=85: "Ultimate Resource", when Hokutozai Minami argues that Ryusui would be the type Tsukasa despises the most.

Fandom
FAN FICTION

TUMBLR

PIXIV

Trivia

 * Tsukasa's name could be translated to officer lion king while Ryusui's means waterdragon of the seven seas. The names allude at two sumptuous beasts that reign over different surfaces of nature, almost a promise of a mythologic duel. It also alludes to the fact that Tsukasa and Ryusui are natural carismatic leaders that persuade others of the effectiveness of their beliefs, however, they defend opposite ideas and practices.
 * Ryusui specifically mentions Tsukasa among men he desires (in addition to women) when he talks about his greed for humanity, and Tsukasa listens and smiles softly.