Momonosuke is the son of Kozuki Oden and Kozuki Toki, born on the ship of the Whitebeard Pirates. As the heir to the Kozuki Clan, he is destined to become the shogun of Wano Country. After his father's execution by Kaido and Kurozumi Orochi, his mother used her Devil Fruit power to send him and several retainers twenty years into the future to protect the bloodline. To conceal his identity during his travels, he initially pretended to be the son of his retainer Kin'emon.

In his early appearances, Momonosuke is a frightened and insecure child who masks his fear with arrogance and demanding behavior, often insisting that others treat him as a superior despite his helplessness. He suffers from a deep sense of unworthiness, feeling he cannot live up to his father's legacy, and is traumatized by Kaido's past assault, which gave him an intense fear of heights. Over time, his personality shifts as he faces the immense responsibility placed on him. He learns to accept his role, make difficult decisions, and act despite his fear. By the end of the Wano arc, he has matured into a determined and self-sacrificing young leader, willing to sacrifice his childhood for the sake of his country.

Momonosuke's central motivation is to fulfill his father's dream of opening Wano's borders to the world and to liberate the country from the tyrannical rule of Kaido and Orochi. He carries the weight of his family's legacy and feels a strong duty to protect the people of Wano. His journey is one of accepting that legacy and growing into the leader his people need.

His role in the story becomes crucial during the Wano Country Arc. He is the legitimate shogun whose return inspires the samurai and allies to rise against Kaido. He also serves as a key link between the Straw Hat Pirates and the forces of Wano. During the raid on Onigashima, the retainer Shinobu uses her Devil Fruit to age Momonosuke's body to that of a young adult, giving him the physical form needed to command respect and wield his powers effectively, though mentally he remains a child. After Kaido's defeat, he formally becomes the shogun and begins the process of opening Wano.

Key relationships include his bonds with the Straw Hat Pirates, especially Luffy. Initially, Momonosuke calls Luffy his servant and tries to assert superiority, but over time he develops genuine respect and trust for Luffy, viewing him as an inspiration and ally. He shares a deep bond with his retainers, particularly Kin'emon, who raised him. His relationship with his estranged sister Hiyori is also significant, as they reunite after decades apart. His father Oden's legacy is a constant presence in his life, driving his sense of purpose.

Momonosuke possesses several notable abilities. He inherits the rare power known as the Voice of All Things, which allows him to communicate telepathically with the ancient elephant Zunesha and sense the intentions of others. He also has the ability to read and translate Poneglyphs, a skill passed down through the Kozuki Clan. After accidentally eating an artificial Zoan-type Devil Fruit created by Vegapunk, he can transform into a massive pink dragon. This fruit is a near-perfect copy of Kaido's Fish-Fish Fruit, Model: Azure Dragon. In dragon form, he can fly, create flame clouds strong enough to lift Onigashima, and unleash powerful bites and attacks. Over time, he also demonstrates proficiency in Observation Haki and Conqueror's Haki. He inherits his father's sword, Ame no Habakiri, one of the Great Grade Blades, but rarely uses it in combat, preferring to rely on his dragon powers. Despite his growing abilities, his lack of experience and mental youth sometimes limit his effectiveness.