"Winch Green" Vinsmoke Yonji is a member of the Vinsmoke Family, making him both a prince of the Germa Kingdom and a commander in its military arm, Germa 66. He is Sanji's and Reiju's younger brother. As noted by the Straw Hats, Yonji heavily resembles his brother Sanji; he has almost identical facial features, including the curly eyebrows common to the Vinsmoke siblings. However, his hair is slicked back into a cowlick, his eyebrows curl in the inverse direction (both face the left side of his face), and he lacks facial hair. Yonji wears all dark clothing, with his shirt bearing the number "66" and his hooded cloak bearing the number "4". He wears a striped belt with the Germa 66 Jolly Roger as a belt buckle, metal gloves, and sunglasses and earphones on the top of his head.
As Nami observed, Yonji, like his older brother Sanji, exhibits some lecherous qualities. He is immediately struck by Nami's beauty, and is seen ogling Nami's bounty poster when Germa 66 depart from their brief meeting with the Straw Hats. In all other respects, however, Yonji is very different from Sanji. He has been described as callous and stingy, refusing to help a dying Luffy and challenging the Straw Hats to plunder the Germa 66 ship for the cure for Luffy's condition. His elder sister, Reiju, claimed that he lacked all humanity. He is also proud, getting angry at Reiju when she humiliated him in front of the Straw Hats. Yonji is also very mindful of his family's affairs, deciding to not get involved with the Straw Hats in order to ensure that Sanji's wedding would not be jeopardized. Being part of the Vinsmoke Family, Yonji possesses political power over the Germa Kingdom, as well the World Government. Yonji's father, Vinsmoke Jajji, claimed that he was a formidable fighter.
However, after a hostile encounter with his older brother Sanji, it is implied that his strength is less than Sanji's. He possesses some degree of durability, as he appeared largely unharmed by his sister Reiju's kick despite her using enough force to send him flying several meters away. He also has quick reaction speed, as he was able to put up a defense against Reiju's unexpected kick. Being from the technologically advanced Vinsmoke Family, Yonji possesses high-tech equipment. He possesses shoes that allow him to levitate in midair. Yonji first encountered Luffy's Sanji Retrieval Team right after they entered Big Mom's waters. Luffy's group mistook him for Sanji until Yonji revealed otherwise. Once he saw Nami, he became love-struck. Seeing that the Thousand Sunny was not the ship Sanji was on, Yonji was about to leave. Chopper called out to him, still calling him Sanji, much to Yonji's annoyance. Chopper begged for an antidote to Luffy's poisoning, but Yonji taunted the group to try to take it by force. His sister, Reiju, appeared and kicked Yonji for his rudeness. After Reiju cured Luffy of his condition, Yonji and Reiju left the Sanji retrieval team alone without making a scene. They also decided to pretend that they never saw the Straw Hats to prevent Sanji's marriage from being called off. He returned to his ship, eyeballing Nami's wanted poster.