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. He is also more muscular than his brothers, who all have comparatively thin builds, and is taller compared to Sanji.
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 symbol as a belt buckle, metal gloves, and sunglasses and earphones on the top of his head.
When outside of his military attire, Yonji wears a light-colored short sleeved shirt with a frilled collar along with a striped belt and plain black pants.
As a child, Yonji's hair was styled with a small cowlick on top of his head, and he wore a light shirt with the number "4" on it.
As Nami observed, Yonji, like his older brother Sanji, exhibits 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 was rotten to the core. Yonji seems to take amusement in instigating or observing conflict, as he offered to lead Sanji to Niji for a fight with a smile and acted similarly when he tried to goad the Straw Hats into attacking him. He is also proud, getting angry at Reiju when she humiliated him in front of the Straw Hats. This proud nature was in evidence again when, upon suffering defeat at Sanji's hands, Yonji was infuriated and swore to make the latter pay for his actions. According to Reiju, he lacks the ability to feel empathy and sympathy.
As a child, Yonji took part in bullying Sanji, joining his two eldest brothers in mercilessly beating and mocking Sanji's compassion in preparing food for others. This shows that he neither tolerates nor values any such trait, which he believed was worthless to royals. In the present day, Sanji himself claims that, despite his hopes to the contrary, all of the members of his family have only gotten worse with time. Yonji has certainly has not changed his ways since his childhood.
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. He also tried to reprimand Sanji after hearing that Sanji was refusing to cooperate with their family's plans for an alliance with Big Mom, but was promptly overpowered by Sanji in retaliation.
When discussing the mission on Broc Coli Island, Yonji expressed interest in making money as he suggested that they could have taken both sides and doubled the profit.
Yonji is very respectful towards his father as he was quick to scold Sanji for defying the wishes of their father and not marry Purin. In return, Judge expressed concern for his youngest son when asking Sanji what he did to Yonji. Like with his two oldest sons, Judge considers Yonji one of his "precious" sons and refused to consider having Yonji marry one of Big Mom's daughters.
Yonji seems to have typical sibling relationship with his elder sister, despite their royal setting. In their childhood, Reiju did not mind Yonji bullying Sanji and even laughed at it. However, Yonji was angry when she humiliated him by kicking him off their ship and calling him a "stingy idiot" after he refused to help the Straw Hats. Despite all this, the two are united with following their father's orders as they did nothing to halt the Straw Hats knowing that it would potentially stop Sanji's wedding.
Yonji gets along with his oldest brother, as he helped him in bullying Sanji when they were children. He also looked forward to the latter reuniting with Sanji when he and Niji arrived on Whole Cake Island. He even smiled when Ichiji mocked Sanji by welcoming him back and reminding him of his status as a failure in their family.
Yonji gets along with his second oldest brother as he helped him in bullying Sanji when they were children. He also looked forward to the latter reuniting with Sanji when he and Ichiji arrived on Whole Cake Island. Later on, at a family meal, he laughed at Niji's mocking remarks directed at Sanji. Yonji did later suggest that Niji was the one responsible for beating up Cosette and offered to take a vengeful Sanji to him. However, he expressed nonchalantly that he does not object to the beating and simply saw it as a natural consequence of Sanji's actions. Yonji was also shocked and concerned when Sanji kicked Niji and soon thereafter smiled when Niji took Sanji down.
In the past, Yonji along with Ichiji and Niji took to bullying Sanji as they viewed him as a weakling and a disgrace to their family for his cooking which they believed was unfit for royals. He even smiled when it was stated that Sanji "died" in an accident. Thirteen years later, upon reuniting with his older brother Yonji told off Sanji for his defying their father only for Sanji to surprise him by overpowering Yonji, leaving him with a damaged face. Yonji was very surprised at Sanji's new strength, wondering how a "good for nothing" like Sanji managed to get the better of him and swore to get revenge on the latter. When Ichiji and Niji arrived at Whole Cake Island, Yonji (with a devious grin) wondered about the look on Sanji's face when he sees their older brothers. While dining with his family, Yonji burst out laughing when Niji informed Sanji that after he left the family, he and the other Vinsmoke brothers often thought about the amusing ways Sanji could of died while on his own. Yonji also offered to bring Sanji to Niji due to suspecting the latter of being the one who supposedly beat the head chef Sanji defended. However, Yonji instead took Sanji to a secret cloning room in the Germa Kingdom where he went onto explain the secret history of the Germa Kingdom before an uncomfortable Sanji cut him off. Yonji was surprised to see Sanji kick Niji in the face but smiled after he was overpowered and watches as Ichiji tells him of his status as the failure in the family hierarchy.
However, despite their history, Yonji was thrilled when Sanji verbally accepted his royal status and attacked and belittled Luffy. Shortly thereafter, he even offered to assist him in fighting Luffy and Nami.
It was noted by Nami that despite their attraction to her, Yonji was nothing like Sanji due to his selfish nature.
Straw Hat PiratesEdit
Yonji encountered the Straw Hat Pirates while they were on their way to retrieve Sanji from Whole Cake Island. Initially, the entire crew mistook him for Sanji due to their similar appearances, until he removed his hood and introduced himself; however, after he became lovestruck at seeing Nami, Chopper called him Sanji again (much to Yonji's irritation). Yonji also rebuffed the Straw Hats' requests to help them cure Luffy of his poisoning, challenging them to try and take the antidote from him by force like a pirate would. This caused the crew to note that, despite their similar appearances and lecherous streaks, Yonji was nothing like his brother.
When Reiju eventually cured Luffy, Luffy thanked her but demanded that Germa return Sanji, causing Yonji to comment that she had saved the life of their enemy. Despite this, Yonji did nothing to halt the Straw Hats' progress, knowing that a confrontation would possibly jeopardize the imminent wedding.
Yonji encountered the Straw Hats again while he and his family were on their way to Big Mom's palace. He was amused to hear Sanji insult them and even offered to help him take care of the crew.
Abilities and PowersEdit
Being part of the Vinsmoke Family, Yonji possesses political power over the Germa Kingdom, as well the World Government. Unlike the members of most royal families, he (as well as his father and siblings) are also the commanders of their kingdom's military, directly leading their forces in combat and other activities.
Yonji's father, Vinsmoke Judge, claimed that he was a formidable fighter. However, after a hostile encounter with Sanji, it is implied that his strength is considerably less than his older brother's; while Sanji was visibly unharmed after their fight, Yonji's face was severely deformed and required treatment.
Like his other siblings, Yonji was given genetic modifications which gave him superhuman abilities. Judge's aide stated that Yonji's strength and agility had matured into the level of adult men by the time he was a young child. He possesses an outer skeleton which gives him 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. After his fight with Sanji, Yonji's face somehow became deformed as though it was a piece of dented metal; a doctor attempted to repair it appropriately, using first a hammer and then a machine press. Neither of these processes induced much pain or any injury in Yonji. 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 was born on the same day as his three brothers, being the fourth son and youngest child of the Vinsmoke Family. Yonji and his siblings were informed as children that they had been given genetic enhancements, and Judge trained them from a very young age in order to develop their superhuman abilities. All of them but Sanji excelled at their exercises. Yonji, Ichiji, and Niji were initially irritated with Sanji's poor progress, but they eventually took pleasure bullying Sanji for his weakness as well as for acting "un-royally" and serving food to rats.
Eventually, Judge had Sanji locked in the Vinsmoke castle dungeon before staging a state funeral and informing the kingdom that Sanji had died in an accident. At the time, Yonji was gleeful to hear about his brother's supposed death. Six months later, Yonji and his remaining siblings continued excelling in their training and were doted on by Judge. Yonji, Ichiji, and Niji occasionally visited Sanji in the dungeon just to continue with their bullying.
After Sanji ran away from the family, Yonji and his brothers often thought of the amusing ways he might have died while out on his own.
Totto Land ArcEdit
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.
At some point upon arriving at Whole Cake Island, Yonji reunited with Sanji. The two had a hostile encounter with Yonji trying to force Sanji to comply with their father's wishes. In response, Sanji then overpowered Yonji, damaging his face in the process. While getting his face treated, Yonji expressed his surprise and bitterness over this defeat. After his face was fixed, Yonji cursed his brother and swore to get his revenge.
Sometime later, Yonji was having a meal with his father and siblings. He was amused when Niji informed Sanji that when he left the family, he and the other Vinsmoke brothers often thought about the humorous ways he could of died while on his own. He was silent when Sanji voiced his contempt for their family's ways and exited the dining hall while Judge was coercing Sanji by threatening Zeff's life. When Sanji suspected Niji for severely injuring the head chef, Yonji offered to take him to their brother. Yonji then led him to a room filled with large containers containing Germa soldiers. As Sanji stared at the Germa soldiers, Yonji explained to him about their father's past partnership with Vegapunk, the research they were conducting, and that the soldiers are actually cloned humans. Ichiji and Niji appeared shortly afterwards and Yonji was shocked when Sanji immediately attacked Niji out of anger. However, Yonji smiled when Niji knocked Sanji down and Ichiji reminded him of his inferior status in the family. After injuring Sanji further, Yonji, Ichiji, and Niji then left the cloning chamber.
Yonji later departed the Germa Kingdom with his father and siblings and head for Big Mom's castle in his family's carriage. On the way, they encountered Luffy and Nami, much to Yonji's surprise. Upon seeing Nami again, Yonji gazed at her in a love-struck manner. When Sanji coldly turned down Luffy's request to return with him, Yonji laughed at this turn of events. As Luffy refused to listen to Sanji, Yonji was about to deal with the Straw Hat captain. Sanji stopped him, volunteering to handle Luffy himself.
- In keeping with the Vinsmoke Family's naming convention, Yon (四) means "four" in Japanese. This is a reference to Yonji's position as the family's fourth-born son.
- Yonji's cowlick resembles Sanji's from the 11th omake, Mugiwara Theatre - Family Time.
- In Volume 61's SBS, a fan joked that Sanji, having changed his appearance by switching the eye he covers, might be Sanji's twin brother named Yonji.