Koy

Koy is the canon ship between the characters of Kai and Joy in Rustage's second One Piece D&D campaign, One Piece D&D: Marines. Although it hasn't officially happened yet, they both share affection towards one another and are more or less confirmed to become a couple down the line. This ship was created after Tekking101 (the YouTuber who plays Kai on Marines), on a stream of Isekai D&D, one of Rustage's other series', first saw the art of Joy (an NPC played by Rustage, the DM) drawn by series artist Kaerru and immediately informed Rustage that he liked the art enough to consider chasing after Joy as a romantic interest. This may have been a holdover from the first campaign where his character was the only one amongst the four not to have a significant other by the end. Still, after this point, and the acute interest that both Tekking and Kai showed towards Joy in the following episodes of Marines, the ship was eventually reciprocated on Rustage's end, thus creating Koy.

History
Kai is a member of the 101st Branch of the Marines, operating in the West Blue. Kai joined the Marines with his pet crab, Belle, mostly just because he thought that it would be a fun job that could help a lot of people, but also to follow in the footsteps of his beloved uncle, Jiro. However, Kai soon learned that not only was the job far more dangerous that he would have expected, but the Marines weren't quite the symbol of justice that the world at large knew them as. These realizations came to him on a mission given to them by their Captain, Bailey Turner, to retrieve a suit of armor at a black market auction that had formerly been in the possession of the Marines. Upon disguising themselves as criminals and entering the auction, Kai and his squad, led by a Marine named Damian, found the armor and began to bid for it, only for the mayor running the auction to fall over, dead. The auction broke out into panic, with each Mafia family present blaming one of the others for the murder. Among these families, the most powerful of them were The Seven Sisters, a group run by Joy and her sisters, of which Joy and her sister, Kip, were there that day to represent. Their real mission was to steal the armor and replace it with a fake, before covering their escape with the murder of the mayor. In the ensuing fight, some of Kai's squad-mates would come into conflict with Joy, who used the power of her Joy-Joy Fruit to overpower them, convincing the undercover Marines that further conflict would be a waste of time as they were sure to lose, and that the Sisters hadn't been proven to commit any crimes thus far, and couldn't be arrested as a result since their actions were technically above board. It was then that the two met, but due to Joy's polite and friendly demeanor, as well as her beauty, Kai failed to recognize her as an enemy, and the two got along well before she left.

Damian, who had created the suit of armor, recognized the fake to be as such, and received information from Capone Bege that the Seven Sisters were most likely the ones behind it, due to their resources and power, and suggested that they continue their undercover operation, getting the help of one of the other Mafia families to take down the Sisters and retrieve the suit. The squad agreed, choosing to join up with The Menu, a crime family run by chef Rordan Gamsey. During this time, Kai began to question the morals and decisions of Damian, a man who would clearly due anything in pursuit of the Marine's justice, no matter who he has to hurt or even kill. Double that with how nice Joy, the "enemy" had been to him and how much he knew he wasn't suited to be a fearless combatant, and Kai began to second guess some things about his place in the organization. This all came to a head when, after fighting Ninny and Demure of the Seven Sisters, they made their way to Malady, who Damian ended up killing during the ensuing battle. Joy and Kip arrived at that point, only to see their sister, killed by Damian, the very man who was the impetus for the Sisters' foundation as Malice, the leader of the group, was a survivor of the Tarako Kingdom genocide in which Damian, wearing the armor that gave him enhanced abilities, slaughtered her parents, as well as many other knights who were just trying to save the princess from the tyrant king. Joy, knowing Kai to be the most sensible and empathetic amongst the group, explained to him that Damian was bad man, that he couldn't be trusted, and that the Marines did more harm than good when they gave power to soldiers like him who would blindly follow their orders. Much to the dismay of the other Marines, Kai (while still very much confused both morally and just in general) sympathized with her efforts, attempting to deescalate the situation and prevent both sides from fighting each other, to which they were willing to agree on until Damian arrived. He and Joy got into an argument that Kai was unable to diffuse, as Damian was just as determined to get the armor back as the Sisters were to give it to him. In the end, Damian and the other Marines, specifically Bradley, took advantage of a devil fruit user's inability to swim and threw both Joy and Kip into the nearby body of water, which prompted Kai, not willing to see anyone else die, to jump in after them. He was even willing to fight and net Bradley, his own friend, to protect them. Ultimately, they were all fished out of the water and Damian proceeded to lecture Kai about his insubordination and how he was letting his emotions get in the way of the mission. Kai, finally coming to terms with just how messed up both the Marines and his own view of the world were, finally snapped, screaming to Damian about how he didn't care about the mission, and that they were all going too far in their pursuits towards the armor, even willing to kill the Sisters to do so. This conflict was quickly resolved through the intervention of Malice, but his choice to protect Joy, who he perceived as innocent, clearly stuck with both Joy and Damian. It was this episode (Episode 9) that any sort of flirtation between Kai and Joy occurred, and it was a huge step forwards in their relationship.

Later down the line, after capturing Malice, Kai was the only one willing to listen to her story and sympathize with her desire to avenge her family and people, to which Malice referred to Kai as a "good man." Kai made the choice not to quit, but rather to help people by staying and fixing the system that resulted in people like Malice and Joy from such extremes in the first place from the inside. Upon returning home, the crew found that Joy was talking to Captain Bailey. It turned out that Malice, predicting her own arrest, had the other Sisters set up as Marine liaisons, which meant that Kai and Joy would continue to see each other from time to time. Right after seeing each other again, Kai received a call from Jiro, his uncle, that informed him of Jiro's suspicions that reports in the Grand Line about a monster were referring to Kai's grandfather, who had eaten a devil fruit and subsequently murdered his grandmother before disappearing. This was the event that made Kai so fearful of Devil Fruits in the first place, and the idea of what had became of his grandfather shook him so much to the core that upon hearing of his return, he dropped the Den Den Mushi and ran away to a balcony nearby. Joy followed him there and the two began to have a conversation in which both of them were clearly infatuated with the other. Kai was nervous to speak to her, both because he liked her and because of what had transpired prior, but Joy cut the tension, explaining that Malice knew what was coming if she went down that path and that it wasn't Kai's fault in the slightest, he did his best and she knew that. Upon him being vulnerable in front of her, lamenting his fear of Devil Fruits and what had happened to his grandfather, she decided to be vulnerable in return. She shared with him a story about how her parents were farmers who had successfully grown a Devil Fruit, it was their pride and joy. Pirates soon attacked the island and killed her parents, but Joy took the fruit because she knew her parents wouldn't have wanted to fruit of their labor to end up in the hands of people so despicable. She ended up eating it herself, using it's powers to kill the pirates. She told Kai that, although Devil Fruits could destroy things, that doesn't mean that they have to. Kai agreed that some of the Devil Fruit users he had met so far turned out to be pretty great, including her, and that she was right to believe that it could do some good. They smiled at each other, and for the first time, she gave him a genuine smile without the use of her Devil Fruit, to which she made a joke that Kai's smile was so beautiful that he must have been the one to eat the fruit. The two laughed and Kai came back with a stronger sense of determination than ever to be a Marine, partially so that he could impress her, but also to enforce the justice that she and her sisters never received from the Marines.