Kozuki Momonosuke is an inhabitant of Wano Country and the son of the late daimyo of the Kozuki Family, Kozuki Oden. He ended up on Punk Hazard after stowing away on a ship. He was first mentioned when Kin'emon told Sanji that he came to Punk Hazard to rescue his "son". His given name was revealed when the samurai was asking if the captive children in the Biscuits Room had seen him, and the clan name revealed when Kin'emon spoke of the child's true heritage. In his first appearance, Momonosuke was seen in his full animal form, an Eastern dragon with a long, pink, slender body. This form has yellow eyes, horns, and scales along his back. In his human form, his appearance matches the stereotype of all samurai found in ancient Japan, with a topknot and the top of his head shaven. As a child, Momonosuke is quite short. Kin'emon used his Devil Fruit ability to give him a pink kimono with peach designs on it, matching his name ("momo" means "peach" in Japanese.) He also wears a red scarf, a purple obi, and a pair of geta.
Momonosuke has a prideful and stubborn personality, as he was unwilling to communicate with the other kidnapped children due to his status as a samurai. He also refused to eat any of the food or candy they offered him. Ironically, this attitude is what prevented him from getting poisoned by NHC10. Despite his pride, he also cried when he thought Kin'emon had died. He claims that he is the man who will become the Shogun of Wano Country. He also claims that he is not afraid of anything and even threw a fit when Luffy asked him if he is afraid of heights. However, he is terrified of Doflamingo after having witnessed his cruelty. Despite his pride and stubbornness, Momonosuke did not deny his fear of him. Momonosuke loves his parents dearly and bears immense hatred and resentment towards Kaido for murdering them. Momonosuke was also utterly frustrated at his powerlessness for not being able to avenge his parents or protecting his retainers. His immaturity and emotional outbursts initially prevented him from assuming authority, but after being spurred on by Luffy, Momonosuke has begun learning to take charge and fulfill his role as the Kozuki Family's leader. Despite his age, he seems quite interested in older women like Robin and Nami, and has used a combination of fake innocence and flattery to get the women to treat him maternally, allowing him to get away with things for which grown men would be considered perverted. While it initially appears he really is just innocent, the look that he gives Kin'emon, Brook, and Sanji makes it clear that he knows what he is doing, clearly showing an extremely perverted nature on his part. Momonosuke loves his parents and could barely speak of their deaths by Kaido and the Shogun of Wano Country. He is very respectful of his father and to the point where he stopped the fight between Nekomamushi and Inuarashi in the name of his happiness. After their deaths, he formed an alliance with the Straw Hat Pirates and Heart Pirates in order to avenge them.
As one of the retainers of the Kozuki Family, Kin'emon swore to protect Momonosuke with his life and pretended to be his father on their journey for the sake of his safety. When Momonosuke was taken by Caesar, he went after him, and even fought one of the Shichibukai in order to save him. Despite his samurai honor, after Kin'emon confirmed that the meal Sanji made on Punk Hazard was fine, he listened and join the feast. Upon his arrival to Zou, the first thing Kin'emon wanted to do was to make sure that Momonosuke arrived there safely. When his true identity was revealed, Kin'emon was shown to carry Momonosuke several times, and even lift him in order to complete his alliance with Luffy, Law, and the Mink Tribe. Despite the father-son ruse, the two do genuinely care for each other almost like family and share an unshakeable bond of trust. Momonosuke even cried heavily when he thought Kin'emon had been killed by the Shinokuni poison gas. Momonosuke also likes to flaunt his success with women in front of Kin'emon and others, which causes his retainer to become bitter and jealous to the point of even speaking ill of his young lord. As the heir of the Kozuki Family, Momonosuke has authority over its five retainers. Due to this, he is one of the main leaders of the Ninja-Pirate-Mink-Samurai Alliance, which would not have formed without him requesting Luffy and Law's assistance as their equal. As two of his retainers are the mink rulers Inuarashi and Nekomamushi, Momonosuke also has sway over the Mink Tribe, a race of powerful warriors who are very loyal to his clan to the point that they would die for any member or retainer. He has the ability to communicate with the giant elephant Zunisha, being capable of seeing through its eyes and giving it orders, which is the only way it can do anything but walk. Should Momonosuke order it to attack, he is in command of a powerful weapon capable of wiping out fleets in a single blow. Momonosuke ate Vegapunk's man-made Zoan-type Devil Fruit that was considered a failure. It transformed him into a serpentine dragon which allows him to seemingly fly, when in actuality he somehow produced clouds that he was able to grab onto and climbed up that way. However, as he has only eaten this fruit recently, he does not seem to have much control over it yet, and in fact transformed unwillingly into his full animal form shortly after consuming the fruit. He did not know that he could produce and climb clouds, as he told Luffy he was stuck in the garbage dump when they met there. When he was frightened by a hunger-induced hallucination, he unconsciously used his cloud ability to escape with Luffy clinging to him.