Like the rest of his family, Niji has light-colored hair and curly eyebrows. He has a similar build to his brother Sanji, being slim with long legs and standing closely to him in height. His hair is groomed into a very large, curved partition across the right side of his face that ends in three prominent spikes.
While commanding the forces of Germa 66, he wears a dark cape bearing the number "2", dark goggles over his eyes, light colored gloves, and a large ascot around his neck. A set of horns extend upwars from a pair of earphone-like devices 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.
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 downright abusing) commoners and servants; he even expresses disgust at Sanji's displays of kindness towards such individuals. 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.
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 Judge.
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 Germa is only ever concerned with who was willing and able to pay them the most money.
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.
As of yet, not much is known about Niji's relationship with his elder sister. However, Reiju was seen smiling at his return to the Germa Kingdom, implying that their relationship is friendly at the very least.
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. 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 of use to their family. He also tells Sanji that, in his absence, the brothers had often speculated on the various amusing ways 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 but was promptly overpowered and mocked by the latter due to his weakness.
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.
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 PowersEdit
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 ranking 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. 
- 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. The henry is the unit of electrical inductance, named after American scientist Joseph Henry.
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 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.
Totto Land ArcEdit
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. Sanji told 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 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. Niji and his siblings later exited the dining hall, and Niji severely beat Cosette in retaliation.
Niji and Ichiji later met Sanji and Yonji in the Germa lab, and Sanji immediately kicked Niji's face in anger over what he did to Cosette. Unfazed by the attack, Niji quickly got back up and beat Sanji into the ground, mocking him for his values.
- 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.