Nanomachines, also known as Nanites, are molecule-sized robots able to modify a body from the inside; which serve as a critical plot device all around the story of the manga Black Cat and its anime adaptation. Creed Diskenth wants to use them in order to become immortal and to create an army of super soldiers. As the story progresses, many types of nanomachines are introduced, always to influence or transform someone's body to improve their physical condition and/or their fighting skills.
Torneo Rudman and Eve
At the start of the manga, the two bounty hunters and main protagonists Train Heartnet and Sven Vollfield set out to capture the weapon dealer Torneo Rudman, who is worth a 50 million yen bounty.
Torneo is leading illegal scientific researches to create genetically modified super soldiers, and created a little girl named Eve, with nanomachines inside her body that heal any wound at a very fast rate and enable her to transform her body at will, for example to turn her hand into a blade.
Train and Sven defeat Torneo and adopt Eve, who becomes one of the main heroes and the only character using nanomachines powers for good. Eve trains her powers to gradually become an exceptional fighter.
She can now manipulate her hair; turn her limb into blades, nano-blades cutting at molecular level, metallic fists, hammers or shields; sprout angel wings to fly and fire feathers; to fully shape-shift; create objects; and send her nanomachines into engines or human bodies.
After his defeat, Torneo admits that he is one of the "sponsors" who funds the Apostles of the Star, and that they both shared the data of their own experiments. Creed himself gets injected with healing nanomachines after having been critically wounded by an explosive bullet from Train.
Later in the story, Creed reveals that his follower Doctor has been conducting experiments on nanomachines, using bounty hunters that they captured as guinea-pigs. When the Chrono Numbers V, VII and XI attack the castle where Creed is hiding, the demented anarchist forces them to fight a brainwashed bounty hunter, who got injected with a type of nanomachine called "Lucifer".
The "Lucifer" transform the bounty hunter into a huge humanoid werewolf with superhuman speed and strength, who can stretch his limbs and elongate his claws to attack. The werewolf also got injected with super-powered healing nanomachines which can cure any wound in a matter of seconds, even if his arm is severed or if his head is destroyed. To defeat this near invincible monster, the Chrono Numbers have no other options that to continuously pummel him faster than he can heal himself, and obliterate him completely.
Later in the manga Creed tries to get rid of Sven by shooting him with a special bullet full of "Lucifer" nanomachines, but Train appears at the very last second and takes the hit. It is then revealed that the "Lucifer" recombine their victim's DNA at random and that no one can predict the transformation beforehand. Train is ultimately turned back into a ten-year-old child by the "Lucifer" and he sets out to turn back to normal.
Train, Sven and Eve ultimately visit the world's best nanomachine expert: Doctor Tearyu Lunatic, who happens to have created Eve from her own DNA. When Train manages to transform back to normal thanks to Tearyu's advices, his body undergoes a final modification that enables him to produce an electrical discharge with his cells, enabling him to fire extremely fast and devastating bullets called the "Rail Gun": a power which he would later use to defeat Creed.
It must be noted that the werewolf Lucifer does not exist in the anime and that Doctor Tearyu appears much later in the anime storyline. Train still gets shot with a "Lucifer" filled bullet. (Shot by Doctor who was aiming for Kyoko Kirisaki, a traitor to the Apostles of the Star.) Train does revert to childhood but he transforms back on his own some time later, after taking a shower.
During the battle of Clarken Island, Doctor captures Toma Fudo and Mundock, two powerful bounty hunters, members of the Sweepers League, and injects them with nanomachines called the "Berserker".
Without a doubt the most awful type of nanomachines in the entire manga, (they do not appear in the anime) the "Berserker" enter their victim's brain and vastly increase the production of substances like dopamine, endorphin and adrenaline, turning the unfortunate victim into a berserker with only one thing in mind: to destroy everything and everyone in sight.
The "Berserker" also increase their victim's strength, speed and reflexes to its maximum, turning them into highly efficient killing machines. Worse, the victims no longer feels pain and fatigue, cannot be knocked out and fight until they get killed or die from exhaustion. Even worse, there is no antidote to the "Berserker" and their victims cannot be turned back to normal or be reasoned by any mean.
Train, Eve and two fellow bounty hunters named River Zastory and Kevin McDougall encounter Fudo and Mundock, who are quickly turned into killing machines and critically wound Kevin, and River shortly after. Train and Eve rush away from their injured comrades and try to find a way to cure the "brainwashed" bounty hunters without killing them. Train ultimately comes up with a plan and he manages to neutralize them both, while Eve sends her own nanomachines into their body to destroy the "Berserker" and cancel their horrid effects.
the Phantom Star Brigade
The Phantom Star Brigade only appears during the final battles of the story, when almost every Tao-wielding Apostle has been taken down. They are the product of the Apostles' ultimate project to create bio-engineered super soldiers.
The Phantom Star Brigade consists in five elite members of the Shooting Star Unit (see below) who were fused with their weapons and turned into cyborgs through the use of nanomachines. It must be noted that they are all merciless murderers who were transformed willingly. Their half-robotic body gives them enhanced strength and durability, and protects them against guns and conventional weapons. They are also equipped with sensors and powerful weapons such as machine guns, rockets and flamethrowers.
There is a “Golem” cyborg filled with countless Gatling and machine guns who gets defeated by Sven; a Flying cyborg who can separate his body in two who gets defeated by Eve; a Snail Cyborg able to hide in his shell to perform a rollout attack, who gets defeated by the Chrono Number VII; a snake-like cyborg who gets defeated by the Number X; and finally a huge four-armed cyborg who gets destroyed by the Numbers IV and VIII.
The last type of nanomachines introduced in the manga and arguably the most powerful, created by Doctor and Eathes (who copied Doctor Tearyu's knowledge about nanomachines) to turn Creed into an immortal God. The "God Bless" can instantly heal any wound and recreate any missing limb out of nowhere in the blink of an eye; they prevent Creed from aging and maintain his body in its peak physical condition, rendering him immune to diseases and illnesses as well.
Yet, the "God Bless" cannot restore Creed's brain. A human brain is so complex that even the most perfect nanomachines cannot fully recreate it. If they were to restore a brain, they would not be able to restore the subject's memory and intellect and thus the subject would only remain with his primal instincts like a wild beast. This was not a problem for the Lucifer Werewolf who was but a killing machine, but this would be too dangerous for a human.
Creed's immortality enables him to defeat the Chrono Number I Sephiria Arks and to gain the upper hand in his battle against Train, but after Creed's ultimate defeat, Eve destroys the "God Bless" with her own nanomachines, ending Creed's mad dreams once and for all.
In the anime, there is one last story arc after Creed's defeat on Clarken Island.
Doctor, Shiki, the Chrono Numbers III, IV and VIII are revealed to be part of another villainous group which worked alongside the Apostles: the Zero Numbers, led by Mason Ordrosso, the renegade Chrono Number XII. It is also revealed that Doctor was Doctor Tearyu's assistant in Eve's creation and that Eve herself was meant to enact a project called "Eden".
This project consists in dousing the entire world, city after city, with a rain of Tao-infused nanomachines that would unify the thoughts of everyone in the world into Eden: a huge, sentient aircraft. This would enable the Zero Numbers to rule the whole world into a dystopia, with every human being's consciousness unified and their bodies used as a fuel by Eden.
The Zero Number kidnap Eve, who is essential for Eden to function, and link her to its artificial intelligence, represented by a young boy in Eve's likeness called Adam. They then destroy Chronos and kill the Elders, and set out to control the world. The Sweepers League joins force with the remaining Chrono Numbers and the reformed Apostles of the Star, to invade the aircraft in order to set Eve free before she gets completely merged with Eden.
The Zero Numbers send thousands of nanomachine-powered monsters, (similar to the Lucifer Werewolf) spawned by Eden after them; before confronting them in person. However, the Zero Numbers are linked to Eden through Tao and nanomachines, which powers them up and resurrect them whenever they get killed.
Train ultimately manages to reach and destroy Eden's core to free Eve, while everyone else is busy fighting the Zero Numbers. The destruction of Eden leads to the death of the Zero Numbers who were all linked to it.