Niji has blue-colored hair and curly eyebrows. Like all the Vinsmoke siblings, his distinctive curly eyebrows form a spiral that resemble the numeral 6. He has a similar build to his brother Ichiji, being slim with long legs and standing close to him in height. His hair is groomed into a very large, curved partition across the right side of his face, ending in three prominent spikes.
While commanding the forces of Germa 66, he wears a blue suit and black cape bearing a blue "2", dark goggles over his eyes, gold-colored gloves, and an aqua ascot around his neck. A set of horns extend upwards from a pair of orange and red earphones he wears, and his belt bears a buckle with a prominent lightning bolt design. Outside of his Germa combat gear, Niji wears a light-colored buttoned shirt with a tie, as well as dark pants belted with a slimmer version of the belt he wears with his Germa gear. For formal functions, he also wears a dark floor-length, single-shoulder royal cape with an upturned collar and a neck clasp.
During the tea party, Niji changed into a dark suit, comprised of a dark double-breasted jacket, a light ascot, dark pants, and dark gloves. He also wore a royal cape similar to the one he was wearing previously, but shorter, light-colored, and clasped together with two chains.
As a child, Niji wore a light shirt with the number "2" on it, as well as a pair of smaller goggles and a smaller ascot. His hair was styled in the same manner as it is in the present, although it was much shorter.
Like most members of his family, Niji is proud of his royal status, looking down upon and even abusing commoners and servants; he even expresses disgust at Sanji's displays of kindness towards such individuals. Since birth, he lacks the ability to feel empathy and sympathy due to the genetic modifications done by his father. He also has no qualms about inflicting pain onto them as shown when he attacked the head chef and even sarcastically stated that he would of beat her anywhere but the face had he known Sanji cared for her. He does not see the importance of food as Sanji does, as he refused to eat a plate of aligot for what he believes to be an unappetizing appearance, and having eaten chocolate beforehand, he would have discarded the food into the trash.
Niji is outspoken and quick to anger, as he was easily provoked by Sanji's insults, and voiced his irritation at seeing that Sanji had failed to greet him and Ichiji upon their return to the Germa Kingdom. He has also exhibited petty and vindictive behaviors, as he was willing to throw a plate at an innocent young chef's face but was stopped by Sanji, causing Niji to become furious that Sanji had defied him. When Sanji ate the meal Niji had wasted from off the ground, Niji was disgusted, claiming he felt like throwing up and went to attack Sanji before being stopped by his father.
While on missions, however, Niji is serious and business-minded, making sure to collect Germa 66's commissions after performing their mercenary services. Unlike his sister Reiju, who displayed an interest in the philosophical issue of whether war or fractured peace was preferable, Niji dismissed her musings by stating that the Germa is only ever concerned with who was willing and able to pay them the most money. Despite his extreme cruelty, Niji is not completely immoral as he expressed slight disdain for Big Mom's habits to keep humans trapped in a book.
Niji is lecherous like his brothers, as he became lovestruck and drooled when he first saw Nami.. Later in the Whole Cake Chateau, he expressed disappointment in the barmaids Big Mom had sent to them and expressed a excited desire to have Nami, disrespectfully referring to her as "it".
Niji's father highly values his "beloved" second son. He approved of Niji bullying Sanji as a child, and refused to consider marrying him off to one of Big Mom's daughters even to seal a powerful political alliance. However, Niji does disregard his father's order to not injure Sanji before his wedding, as he goes on to beat him along with Ichiji and Yonji.
As of yet, not much is known about Niji's relationship with his elder sister. Reiju was seen smiling at his return to the Germa Kingdom, implying that their relationship is friendly at the very least.
However, unbeknownst to Niji, Reiju secretly despises him along with the rest of their family (except Sanji) for their murderous ways and she intends to do nothing to stop the Big Mom Pirates from killing him, believing that there's no place in the world for any of them.
As a child, Niji, alongside Ichiji and Yonji, used to bully Sanji for his "un-royal" habit of cooking. Like his brothers, Niji took pleasure in giving Sanji a serious beating. Once during a training regimen, Niji gleefully attacked his younger brother mercilessly and only stopped when a Germa soldier held him back. When he heard that Sanji "died" in an accident, he was the only one who seemed curious over whether this was true but after finding out Sanji was alive he continued to bully him with Ichiji and Yonji. Thirteen years later, he expressed annoyance at the fact that Sanji did not greet himself and Ichiji upon their return to the Germa Kingdom. As for Sanji, he trembled slightly at their return, implying that Sanji may retain traumatic memories from their abuse.
After reuniting with Sanji, Niji mocks him by voicing his surprise at the fact that Sanji ultimately proved to be of use to their family. He also tells Sanji that, in his absence, the brothers had often speculated on the various amusing ways and place in which Sanji may have died while living on his own. While Sanji is indifferent to this mockery, the two brothers did engage in a major argument immediately afterwards over their different attitudes towards food and respect for commoners. Though he does acknowledge Sanji as his brother, he was disgusted and enraged by the fact that somebody who shares his blood could be so kind to non-royalty as well eating scraps from the floor, even to the point where he's willing to attack him for it. When he was restrained by their father, Niji told Sanji he brought nothing but shame to their family as Sanji stated by being affiliated with them brought shame to him. Sanji also wanted to confront Niji for supposedly beating Cosette after he defended her from him earlier. When Sanji saw Niji again, he immediately kicked him in the face, determined to make him pay for his actions and succeeded in dislocating Niji's jaw but Niji fixed his face and overpowered Sanji by using his "weakness" against him. Niji joined Ichiji and Yonji in welcoming Sanji back to the family before brutally beating Sanji while he lay on the floor.
Like his brothers, he was pleased and amused when Sanji attacked and insulted Luffy despite their friendship, as he can be seen grinning widely at Sanji's comments. However, once Sanji cried at Luffy's words, Niji simply laughed at his anguish along with Yonji.
Niji stated he was looking forward to Sanji living with the family again after Judge explained his intent to take Pudding with them to the Germa Kingdom. Niji later patronizingly toasted Sanji for being useful. On the day of the ceremony, Niji inquired if Sanji tried to escape the wedding only to be told that Sanji was prepared. for the wedding.
Niji's youngest brother used to accompany him and Ichiji as they bullied Sanji, showing that they at least got along. He was also excited at the prospect of a confrontational reunion between Niji, Ichiji, and Sanji. Yonji tends to laugh hysterically at Niji's various rude comments and jokes about Sanji just as he did when they were children, and on one occasion, they both share a laugh at his expense after witnessing him crying.
Niji's subordinates in Germa 66 revere him as one of their commanders, cheering him and Ichiji as they returned from their brief campaign in Broc Coli Island. As a member of their royal family, each soldier is also willing to lay down their life for Niji if commanded to do so.
Niji has no respect in the slightest for servants and commoners and seems to enjoy abusing them, as he threw a plate at Cosette's face and was furious when Sanji intervened.
Abilities and Powers
As a prince of the Germa Kingdom, Niji has political authority over its subjects, and great influence over the World Government. He also holds a high rank within Germa 66, possessing authority over common soldiers who will die for him on command. A portion of Germa 66, under Niji and Ichiji's command, was able to end a two-year war in four hours, thoroughly annihilating the side they were hired to fight.
As a member of the Vinsmoke Family, Niji also has access to the advanced technology developed by his family as well as their extravagant wealth.
He was shown to have knowledge of swordsmanship in his youth, overpowering Sanji several times.
As a result of genetic enhancements, Niji possesses tremendous superhuman abilities. He has an outer skeleton that made him impervious to a blunt sword strike from Sanji, and one of Judge's aides said that he had developed the strength and agility of an adult man while he was a young child. One of his kicks generated enough wind force to knock a by-standing Cosette off her feet, despite Niji halting the kick in mid-air before it connected with Sanji. He is extremely durable, as he quickly fixed his broken jaw after taking a direct hit from Sanji's powerful kick and stood up to fight back almost immediately. He has tremendous tolerance for alcohol, as he and his brothers continued to drink large quantities unaffected while the barmaids all fell intoxicated.
- Stealth (ステルス Suterusu?): Niji escapes his target's sight before quickly appearing right behind them. It was first used in his scuffle with Sanji.
- Henry Needle (起電 (ヘンリー) ニードル Henrī Nīdoru?, literally meaning "Electromotive Needle"): Niji electrifies his knee and knees his opponent in the back. It was first used in his scuffle with Sanji.
Niji was born as the second son and the third child of the Vinsmoke Family. At the time of his birth, the Germa Kingdom was still in North Blue. As a young child, Niji and his siblings were informed by their father that their bodies had been genetically modified, and their father instructed them to train hard in order to develop and take advantage of these superhuman enhancements. All of them succeeded in developing except for Sanji. Niji and his two other brothers were initially irritated with Sanji's poor progress, but they eventually took pleasure in bullying Sanji for his weakness as well as for feeding rats.
When Judge had Sanji locked in a dungeon and then claimed that he died in an accident, Niji wondered if Sanji had actually died. After finding Sanji, Niji and his two other brothers occasionally visited him just to continue with their bullying.
After Sanji left the family, Niji and his siblings often mused about Sanji. However, their discussions typically revolved around the numerous amusing ways that each of them could imagine as to how Sanji may have died while living on his own.
Whole Cake Island Arc
Niji and Ichiji led a platoon of Germa 66 soldiers to end a two-year war on Broc Coli Island. They did so in four hours, and Niji collected the payment for their services from the winning side. Niji and Ichiji then stood on a rooftop to observe the burning country, as they arranged to make the two day journey to Whole Cake Island. While Ichiji expressed excitement at seeing Sanji again, Niji called him a liar.
Two days later, Niji and Ichiji arrived at the Germa Kingdom. As they received a hero's welcome from the Germa 66 soldiers, Niji was annoyed that Sanji was not there to greet them, but was told by Ichiji to quiet down.
Sometime later, Niji was having a meal with his father and siblings. After a discussion with his father about the Reverie, Niji stated that he thought he would never see the day Sanji would be useful. Niji told Sanji that when he left the family, he and the other Vinsmoke brothers all spoke about him but their conversations revolved around the amusing ways and place he'd most likely to die. Sanji ignored this and proceeded to tell Niji to finish what was on his plate, but Niji refused and was irritated when Sanji called him a spoiled prince who does not know the value of food. Niji then angrily called the head chef, Cosette, and threw his plate at her. Sanji caught the plate and scorned Niji for trying to hurt a woman. As Sanji ate the food that fell on the floor, Niji was disgusted at this. Niji attempted to kick Sanji, but their father quickly stopped him. Niji told Sanji he brought shame to their family, but Sanji stated being affiliated with them is shameful to him. As their father coerced Sanji by threatening Zeff's life, Niji and his siblings exited the dining hall, and Niji severely beat Cosette in retaliation for Sanji's actions.
Niji and Ichiji later met Sanji and Yonji in the Germa lab, where they mocked Sanji for still being pathetic. Seeing Niji again, Sanji immediately kicked Niji's face in anger over what he did to Cosette. Niji's jaw was broken, but he fixed it with his hands and quickly got back up and beat Sanji into the ground when Ichji reminded Sanji of Zeff. While Sanji was lying on the floor, Niji mocked him for his values.  After injuring Sanji further, Niji left the lab with Ichiji and Yonji.
Niji later departed the Germa Kingdom with his father and siblings and headed for Big Mom's castle in his family's carriage. On the way, they encountered Luffy and Nami. Upon seeing Nami, Niji gazed at her in a love-struck manner. When Sanji coldly turned down Luffy's request to return with him, Niji smiled.
Niji watched with amusement as Sanji continued to reject Luffy and Nami and then severely beat Luffy by repeatedly kicking him. After knocking Luffy down, Sanji returned to the carriage and the Vinsmoke Family continued on to the Whole Cake Chateau. When Luffy declared that he would wait for Sanji, Niji laughed alongside Yonji.
As the carriage approached Big Mom's castle, Niji expressed amusement at watching an army marching towards Luffy and Nami's location. After arriving at the castle, Niji and his family had a meal with Big Mom and Pudding.
Sometime after the lunch meeting, Big Mom took the Vinsmoke Family (except Sanji) to her library and showed them her collection of rare creatures stored in books.
Sometime after the tour, Niji, Judge, Ichiji, and Yonji had a private discussion in the Vinsmoke guest room. Judge told his sons that Pudding would be living with them on Germa Kingdom after the wedding so they could use her as leverage against Big Mom. When Niji asked if Big Mom would use a similar tactic, Ichiji told Niji that it would not have any effect on the Vinsmoke Family if Sanji's life was threatened. Niji then expressed excitement at the thought of Sanji living with them again.
During the night before the wedding ceremony, Niji, Ichiji, Yonji, and Judge celebrated the anticipated sealing of their alliance with Big Mom. While they drank, they toasted to Sanji for being useful.
On the day of the wedding ceremony, Niji was shown getting dressed for the wedding, alongside his father and elder brother. As Judge and Ichiji inquired about Reiju's location, Niji went to wake Yonji up and told him to get prepared for the tea party. An hour later, the Vinsmokes headed to the ceremony venue on the roof of the Whole Cake Chateau. As they made their way through the castle, Niji told Reiju off for criticizing their loud drinking the night before. After Niji asked if Sanji tried to escape, the Vinsmokes received confirmation from one of Big Mom's subordinates that Sanji was ready for the ceremony.
- Ichiji, Niji, and Yonji vs. Sanji
- Niji vs. Sanji (Germa Kingdom training ground)
- Niji vs. Sanji (Soldier Depot)
- In keeping with the Vinsmoke Family's naming convention, Ni (二) means "two" in Japanese. This is a reference to Niji's position as the family's second-born son, despite being the third-born child.
- In the Marine comic strip, Sora, Warrior of the Sea, there is a brief scene of Sora facing Germa 66. On Germa's side are three cloaked agents, one of whose cloaks bears the number "2"; the figure is likely a representation of Niji.
- Niji's callous disregard of food and women are in total contrast to his younger brother Sanji's values, who endears both.
- Niji's attack known as Henry Needle referenced to a henry, which is the unit of electrical inductance, named after American scientist Joseph Henry.