Ichiji has red-colored hair and curly eyebrows. Like all the Vinsmoke siblings, his distinctive curly eyebrows form a spiral that resemble the numeral 6. He is slim with long legs and stands at a comparable height to his brother Niji, being slightly taller. His hair is fashioned into three prominent cowlicks on the right side, and also curls upwards slightly at the roots, like his sister's. His right arm bears a prominent tattoo: the number "1" in the center of a stylized circle, surrounded by flames.
When commanding the forces of Germa 66, he wears a dark red suit, white gloves, a red ascot, a white cape (bearing a red "1" on its left), as well as a pair of sunglasses. Outside of his Germa combat gear, Ichiji wears a dark shirt with an upturned collar and a light cravat, as well as a pair of light-colored pants. For formal functions, he also wears a floor-length, single-shoulder royal cape with an upturned collar and a neck clasp. However, unlike his siblings, his cape is light-colored.
During the tea party, Ichiji changed into a light frilled dress shirt, as well as dark pants and dark gloves. He also wore a royal cape similar to the one he was wearing previously, but shorter and clasped together with two chains.
As a child, Ichiji wore a dark shirt with the number "1" on it. His hair was styled in the same manner as it is in the present.
Like most members of his family, Ichiji is proud of his royal status, looking down upon commoners and servants. Like his father, Niji, and Yonji, he 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. Ichiji is also cold and brutal to the people he has been paid to fight against, caring only about his task and not their lives. He does not like it when a battle takes a long time, and expresses his disdain for the people who oppose him. He also expects his subordinates in Germa 66 to lay down their lives for him at any point, showing no emotion at their deaths. While Ichiji seems to be excited to see his brother, Sanji, and attend his wedding, Niji stated that he was lying.
In his childhood, he (like his other brothers) cruelly bullied his younger brother Sanji for showing compassion to non-royals. While Ichiji, Niji, and Yonji all enjoyed tormenting Sanji, Ichiji seemed to enjoy it the most. Notably, Ichiji scowled in anger as he beat and ridiculed Sanji, rather than laughing and smiling like his brothers and sister. 
Though Ichiji told his younger brother Niji to pipe down when Niji was annoyed that Sanji was not greeting them as they returned to the Germa Kingdom, this friendly attitude did not last long as when he came face to face with his younger brother, Sanji. Ichiji became cold and hostile when Sanji stated that his family offended him on every level. Ichiji replied that Sanji was a disgrace, clarifying that "royals should act like royals".
Ichiji's father highly values his "beloved" eldest son. He approved of Ichiji 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 Ichiji'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 Ichiji, Reiju secretly despises him, along with the rest of their family (except Sanji), because of his murderous ways. She intends to do nothing to stop the Big Mom Pirates from killing him, believing that there was no place in the world for any of them.
Ichiji appears to get along well with his younger brother, alongside whom he has co-commanded Germa 66 forces in combat situations. Niji complies without a response when Ichiji tells him to quiet down on their return to the Germa Kingdom. This could imply a slight amount of deference as the eldest brother, given Niji's confrontational behavior.
As a child, Ichiji, alongside Niji and Yonji, used to bully Sanji for his "un-royal" habit of cooking. He was shown to be irritated by Sanji's kind and compassionate behavior such as caring for a little pet turtle. Ichiji even stated that, despite sharing the same birthday, Sanji was a disgrace to the family. When it was said that Sanji died, Ichiji was relieved that he was gone as he was useless. Even when Sanji was condemned to be imprisoned forever by Judge, Ichiji refused to let him off and continued to visit his cell with his other two brothers just to continue bully him. After completing a mission on Broc Coli Island, he stated that he could not wait to see his younger brother again, but Niji called him out as a liar. Furthermore, he seemed indifferent to the fact that Sanji did not greet himself and Niji upon their arrival for his wedding. As for Sanji, he trembled slightly at their return, implying that he may retain traumatic memories of their abuse.
After reuniting with Sanji, their disagreement over the proper behavior of royals was immediately reignited. While Sanji denounced their family's spoiled attitude and cruel treatment of servants, Ichiji once again labeled Sanji a disgrace and stated that he had never understood how Sanji's brain worked, even when they were children.
When Sanji attacked Niji, Ichiji distracted him by reminding him of his weakness which allowed Niji to overpower Sanji. As Sanji laid on the ground, Ichiji took the opportunity to mockingly "welcome" him back to the family and tell him his status as a weakling in the hierarchy.
However, despite their differences, he was pleased when it appeared that Sanji accepted his royal status. He was seen smiling to himself when Sanji insulted and attacked Luffy, despite their friendship.
Ichiji still bears only cruelty towards his brother as he mentioned that he and the rest of family will be unaffected if Big Mom takes Sanji hostage.
The night before the wedding, Ichiji and his family expressed their "gratitude" to Sanji for making their successful future imminent and shared a toast to him while still calling him a failure.
Yonji used to accompany his two oldest brothers in bullying Sanji, showing that they got along at the very least. He was also excited at the prospect of a reunion between Ichiji, Niji, and Sanji.
Ichiji's subordinates in Germa 66 revere him as one of their commanders, cheering him and Niji 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 Ichiji. One soldier was shown doing exactly this during that campaign, after Ichiji ordered him to take a bullet intended for him.
Abilities and Powers
As a prince of the Germa Kingdom, Ichiji has political authority over its subjects. He also holds a high ranking within Germa 66, giving him authority over common soldiers as well as the ability to order them to sacrifice themselves in his defense. A portion of Germa 66, under Ichiji and Niji'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, Ichiji has access to the advanced technology developed by his family, as well as their extravagant wealth.
Along with the rest of his siblings, Ichiji was enhanced with genetic modifications that give him tremendous superhuman abilities. When Ichiji was a child, Judge's aide stated that his strength and agility was on the level of an adult man. He developed an outer skeleton that enhanced his durability; his face made a metallic noise when Sanji punched it, and the force of impact hurt Sanji's fist. He also possesses great physical strength, as he was able to grab the arm of an enemy soldier and twist it around so the soldier was pointing his gun at himself, keeping the gun stationary as he talked, although the enemy soldier was weakened from fighting. He has tremendous tolerance for alcohol, as he and his brothers continued to drink large quantities unaffected while the barmaids all fell intoxicated.
Ichiji was born in the Germa Kingdom on the same day as his younger brothers, while the Kingdom was still in North Blue. When Ichiji and his siblings were very young, Judge informed them that he had enhanced their bodies with genetic modifications, and instructed them to train so that their bodies would develop superhuman abilities. The siblings excelled at their exercises, except for Sanji who continually lagged behind. Ichiji, Niji, and Yonji were initially irritated with Sanji's poor progress, but they eventually took pleasure in bullying their brother for his weakness as well as for giving food to rats. During one of their bullying sessions, Sanji attempted to punch Ichiji to get his food back, but Ichiji retaliated by beating him viciously.
When Judge had Sanji locked in a dungeon and informed the kingdom that he died in an accident, Ichiji expressed indifference, since Sanji had been "useless" anyway. However, Ichiji, Niji, and Yonji eventually learned the truth that Sanji was still alive and occasionally visited him in the dungeon to continue bullying him.
After Sanji left the family, Ichiji and his siblings often mused about Sanji. However, their discussions typically revolved around the numerous amusing ways that each of them could conjure as to how Sanji may have died while living on his own.
Whole Cake Island Arc
Ichiji and Niji led a platoon of Germa 66 soldiers to end a two-year war on Broc Coli Island. Ichiji and his men ruthlessly slaughtered several enemy soldiers, and a lone survivor attempted to shoot Ichiji. On his command, however, a Germa 66 soldier stepped in front of Ichiji and took the bullet. Ichiji then confronted the enemy soldier, getting angry that the soldier was making him late for his brother's wedding, and shot the enemy soldier with his own gun. Ichiji and his men walked away, having won the war. Ichiji and Niji then climbed atop a building as they contacted an unknown person, preparing to make the two-day voyage to Whole Cake Island. Ichiji expressed anticipation at reuniting with Sanji, but Niji called him a liar.
Two days later, Ichiji and Niji arrived at the Germa Kingdom. As the Germa 66 soldiers gave them a hero's welcome, Niji was angry that Sanji was not there to greet them, but Ichiji told him to quiet down. The brothers soon reunited with the rest of their family, including Sanji, while dining. After Sanji defended the head chef from Niji, Ichiji scorned Sanji for going against their family's ideals. Ichiji later exited the dining hall as Judge coerced Sanji by threatening Zeff's life.
Ichiji and Niji later met Sanji and Yonji in the Germa lab. When Sanji kicked Niji in the face, Ichiji reminded Sanji that they knew about Zeff. After Niji got back up and knocked Sanji down, Ichiji sarcastically welcomed Sanji back home and reminded him on his inferior status in the family. Ichiji and his two younger brothers then proceeded to injure Sanji further before leaving.
The Vinsmoke Family later departed the Germa Kingdom and head for Big Mom's castle in their carriage. On the way, they encountered Luffy and Nami. Upon seeing Nami, Ichiji gazed at her in a love-struck manner. When Sanji coldly turned down Luffy's request to return with him, Ichiji smiled.
Ichiji then watched with a more serious look 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.
After arriving at Big Mom's castle, Ichiji 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, Ichiji, Judge, Niji, 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.
During the night before the wedding ceremony, Ichiji, Niji, 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, Ichiji was shown getting dressed for the wedding, alongside his father and Niji. He and Judge inquired about Reiju's location, before being assured by one of Big Mom's subordinates that she was getting prepared in a different room. An hour later, the Vinsmokes headed to the venue for the ceremony, on the roof of Whole Cake Chateau. When Big Mom arrived to commence the banquet, Ichiji stood to the left of Judge, holding a wine glass.
- In keeping with the Vinsmoke Family's naming convention, Ichi (一) means "one" in Japanese. This is a reference to Ichiji's position as the family's first-born son, despite being the second-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 "1"; the figure is likely a representation of Ichiji.
- The colors of Ichiji's clothes are inverted, relative to those of his siblings. For example, instead of a light shirt with dark trousers, he wears a dark shirt with light trousers. Unlike his siblings, who wear dark cloaks with both their Germa raid suits and their royal attire, Ichiji's cloaks are light colored.