Modified humans are humans who have been biologically modified through genetic enhancements to have enhanced physical traits. So far, the only known modified humans are four of the five children of the Vinsmoke Family.
Modified humans are normal humans with artificial enhancements made at a genetic level. Their external appearances remain identical to that of normal humans. However, their genetic enhancements also caused a change to their hair color, as none of the Vinsmoke children (except Sanji, whose modifications were negated) inherited their parent's hair color.
Modified humans are humans whose Lineage Factor has been manipulated, usually at the embryonic stage. By manipulating this "blueprint of life", the growth of the life form in question can be changed accordingly.
Vinsmoke Judge engineered his four sons to possess superhuman traits, by applying the procedure to them during their fetus stage, against the wishes of his wife, Vinsmoke Sora. By doing so, he ensured they would develop a secondary, hardened skeleton known as an Exoskeleton (外骨格 Kaikokkaku?), as well as other physical enhancements. However, while Ichiji, Niji, and Yonji all adapted to their father's engineering, Sanji did not gain any of the genetic enhancements due to their mother taking a powerful drug intended to negate the surgery's effects. The drug rendered Sanji alone a "normal human".
Reiju, although not born alongside her four brothers, was also modified by her father to possess such enhancements, but with some alterations: she cannot disobey direct orders, but retains her emotions and can circumvent his orders with loopholes.
Modified humans broach a controversial topic about the morality of genetic engineering, its benefits and drawbacks. Judge, a warrior king, was adamant about turning his four sons into superhuman commanders for the family's army, Germa 66, while his wife Sora was against the negation of their ability to feel compassion. Sora fiercely believed that such modifications would render their sons inhuman, to the point that she consumed a powerful drug in an attempt to negate their effects; this drug only worked on Sanji, and conversely caused permanent damage to Sora's health that lead to her premature death eight years later. Reiju, three years older than her brothers, was also subjected to a different variant enhancement which similarly give her superhuman physical potential and an exoskeleton. However, the procedure interfered with her emotional capacity in a different manner; she cannot disobey direct orders, but retains her natural compassion and can resist or circumvent her father's commands with loopholes.
Due to their lack of inherent compassion in conjunction with their royal and militant upbringing, the three modified brothers of the Vinsmoke Family look down on others with utter haughtiness, and are willing to kill and injure in cold blood. This includes wanton acts of cruelty, such as ordering soldiers to sacrifice their own lives for them, or brutally attacking a devoted chef to vent out dinner frustrations. Their cruelty extends to their mutual quadruplet brother, Sanji, whom they see as a failure due to his lack of genetic enhancements and his capacity to empathize. Beyond this cruel and self-superior nature, the three brothers are generally less emotional than a normal human being; in particular they have no personal fear of injury or death, traits intended to make them effective warriors. Judge's goal was for this trait, in tandem with a calculating efficiency to avoid injury purely out of pragmatism only, to render them master combatants and killing machines, but to his horror it proves to be a great flaw. When the Vinsmoke family were surrounded by the Big Mom Pirates and restrained at gunpoint without arms or effective armor, the modified brothers quickly assessed that, having no means to retaliate, defend, or escape their predicament, there was nothing for them to do; the family had simply lost, and so they sat there unconcerned, accepting defeat and death nonchalantly. The trio joked about how badly they had been deceived; and when their father, who was more naturally upset over the impending death of himself and his children, began to plead and cry, they coldly deemed him a failure in contrast with their usual respect. Not even their own beloved father's sentiment mattered to them in that dire situation. Judge, witnessing this flawed design in their genetic engineering, desperately thought at last that something was wrong with them.
The modified humans engineered by Judge all possess superhuman physical strength. During their childhood, with training, the four Vinsmoke siblings' strength and resilience rapidly increased to the point of equalling an adult man's. Further administration of drugs can also further the augmentations.
The Vinsmoke siblings also possess an exoskeleton which grants them enhanced durability, leading them to be renowned for their "iron bodies". They also possess an increased healing rate, as Reiju managed to rapidly heal after her leg was shot, and was even able to engage in combat a few hours later.
Reiju, Ichiji, and Niji have demonstrated special abilities which resemble the use of Devil Fruits; it can be presumed that Yonji possesses a similar unique ability. The abilities which have been demonstrated are tied to the user's epithet: Reiju's ability ("Poison Pink") allows her to suck out powerful poisons out of an afflicted individuals and neutralize them within her body, demonstrating complete immunity to the deadliest poisons. She is also able to secrete poison to attack enemies, an ability which was unique to the Doku Doku no Mi. Niji's ability ("Dengeki Blue") allows him to generate electricity from his body, similar to the Goro Goro no Mi. Ichiji's ability ("Sparking Red") allows him to generate explosions from his body, similar to the Bomu Bomu no Mi.
The Exoskeleton, while powerful, is not invincible. A powerful enough force can cause it to dent, although it can be repaired with a counter blunt-force, such as a hammer or a "Press Machine". Should something like the jaw be dislocated, the modified human in question can rearrange it by hand. The Charlotte Family also developed special "Candy Jacket" bullets capable of piercing through Reiju's exoskeleton, causing her left leg to bleed profusely.
Additionally, there are notable limitations to the modifications' accompanying impact on emotional capacity. While Judge originally perceived Ichiji's, Niji's, and Yonji's lack of emotions as a strength in combat, it proved a limitation when they responded indifferently to the prospect of imminent death. As Reiju retained her emotions and humanity, she grew to despise her father and brothers (except Sanji), and was forced to carry out her father's orders in a way which conflicted with her innate moral nature. However, while Judge's modifications on Reiju prevent her from directly opposing him, they do not appear to prevent Reiju from undermining his orders indirectly, as she replaced Sanji's exploding wrist bracelets with fakes. Reiju was also able to choose not to warn her family about Big Mom's imminent plan to betray and kill them, showing that she retains a large degree of autonomy. Ironically, while Judge had always viewed Sanji as a failure due to his compassion and humanity, it was this very compassion that led Sanji to rescue his family despite the years of mistreatment he received at their hands, an act that shocked Judge and moved Reiju.
It appears that genetic modifications can only remove a limited range of natural human emotions and cannot alter the subject's natural personality. Despite their loss of "soft" emotions such as empathy and compassion, Ichiji, Niji, and Yonji are still capable of feeling such emotions as anger and cruel delight, and they all exhibited a lecherous streak at the sight of Nami.