Naraku's incarnations were created to aid him in his battle against Inuyasha and his group as well as keeping a better eye on Naraku's surrounding situations. Naraku was constantly releasing detachments of himself to carry out his bidding. Only a few of the detachments truly lived up to their purposes as most betrayed him or were quickly disposed of by the people going after him.
To keep his control over his incarnations, Naraku usually held their hearts within his own body. By squeezing their hearts, he was able to inflict them pain, and by destroying them he could kill them off altogether. Only three of his detachment were not controlled in that manner, namely Musō, Akago, and Hakudoushi, since they were actually manifestations of Naraku's human heart and (except for Hakudoushi) they carried their (and his) heart within themselves. In the manga, he is also never shown manipulating Kanna's heart; this may be due to the fact, oft-repeated by Kagura, that Naraku trusted her completely. But it is also possible that Kanna, as the personification of Nothingness, did not possess a heart of her own; this is in fact what Naraku says of her after just her death. In the anime, by contrast, in Kanna's final battle, he's shown holding her heart, which has a crystal-like appearance, and then destroying, effectively using her and her mirror monster as bomb to kill Inuyasha's group.
Naraku gained the ability of producing detachments from his body after Kikyō handed him a large fragment from the Shikon no Tama that she had stolen from Kagome. The process by which they are created, however, is never clearly explained. In the manga, there are shown lumps of Naraku's flesh disposed inside of large pots, from which mature incarnations will eventually spring forth.
Naraku's incarnations do not appear to correspond to any one demon absorbed previously by him. Instead, they may be a miscellany of his demon and human parts. In fact, several of Naraku's incarnations (namely, Kagura, Goshinki, Musō, and Hakudoushi) are shown to possess the same spider scar on their backs as Naraku, which is presented as proof of his tie with the heavily scarred Onigumo. Onigumo's feelings, including his love for Kikyō, also appear to exist within Naraku's incarnations, even if they never manifest any feeling for her with the exception of hostility or weariness in face of her powers. This is proven in Chapter 174 of the manga, when a stillborn detachment of Naraku's attacks Kikyō and automatically implodes, because, as Kikyō explained, she had used a spell whereby Onigumo's feelings for her, which exist inside of Naraku and his incarnations, would come to her aid whenever they tried to touch her.
Though some of his incarnations rebelled against his rule and plans, and desired to live free from his presence, it seems this would not be possible, and that the moment Naraku should killed, they would automatically die with him. This is proven by the fact that, after Naraku's main body is pierced by Kagome's purification arrow during the final battle, losing an arm, Byakuya, his one living incarnation, loses the same arm himself. Plus, after Inuyasha dispatches Byakuya to the Netherworld with the Meidō Zangetsuha, Byakuya calmly says in his final moments that he has no regrets, since he would die the moment that Naraku (himself already on his death throes) perished.
Kanna was an albino girl that barely spoke. She was a demon of mu, "void" or embodiment of nothingness; which also explained why she had no demonic aura. Kanna comes from an old word meaning "godless world." She was Naraku's first detachment, and remained utterly loyal to him while knowing that they were just used as tools in his schemes. Representative of the void in which she controlled, Kanna herself was completely devoid of any emotion, and never questioned any orders that were given to her by Naraku. In Chapter 480, after a failed attempt to destroy Inuyasha's Tessaiga, Naraku ordered her to commit suicide by shattering herself—as a final attempt to destroy Inuyasha's group (in the anime, he destroyed her heart). In her final moments, however, Kanna seemed to display some signs of emotions, as if she really did not wish to die. It was in her death that she committed her only act of defiance to Naraku: As a shard of Kanna's shattered body flew into Kagome's eye, Kanna used this opportunity to show Kagome the light that was present within the Shikon no Tama, which was the key to defeating Naraku.
Kagura was a powerful wind sorceress. She was the first of Naraku's detachments to appear in the manga and the anime, though by order of birth, Kagura was actually his second detachment. Naraku endlessly used her as bait to test his enemies' strength and, aware of her betrayals, kept good watch over her. Her storyline was constantly followed as she plotted to destroy Naraku and win back her heart, often in combination with Sesshōmaru, to whom she was attracted. Eventually, Kagura was killed by Naraku after she openly betrayed him to protect Kohaku.
The third detachment of Naraku, Goshinki was a large horned ogre with the ability to read minds. He was extremely agile, physically very powerful, and had teeth so strong that they ultimately broke the Tessaiga. That, however, ended up being his own undoing. As happened whenever his sword was taken away from him in life-risking situations, Inuyasha transformed into a full yōkai out of desperation and thus quickly destroyed Goshinki. Following his death, Sesshōmaru contracted Kaijinbō to create Tōkijin from the fangs of Goshinki.
Jūrōmaru was the fourth detachment and brother of Kagerōmaru, who lived in his stomach. Unlike the other detachments, Jūrōmaru lacked any intelligence, and acted simply as a feral beast that could only be controlled by his brother. Indeed, Naraku kept him shackled to maintain some control over him. Both he and Kagerōmaru loved to feast on the insides of those that they killed. He and his brother Kagerōmaru were both killed in battle by Inuyasha with Tessaiga.
Kagerōmaru was a small, fast and intelligent detachment from Naraku, proving to be a worthy adversary to the wolf demon Kōga. Unlike most detachments, he was born with his brother at the same time. He was not loyal to Naraku, and even tried to kill him by decapitation and had his heart squeezed as punishment before being sealed in his younger brother's gut. Like Jūrōmaru, he loved to feast on the innards of those he fought. Kagerōmaru's appearance bared similarity to a parasite, which takes residence in other people. He further proved this similarity by being able to hide within Jūrōmaru's stomach for sneak attacks. He and his brother, Jūrōmaru were both killed in battle by Inuyasha with the Wind Scar.
An unknown yōkai was created by Naraku in Chapter 174 in the manga, after Kikyō destroyed Naraku's right arm. Its body structure was similar to Kagerōmaru's. Out all the incarnations Naraku created, this one had the shortest life - only a few moments. It slithered out of one of the pots containing Naraku's flesh and attempted to wrap its tongue around Kikyō's ankle. However, at the very moment that it did, it disintegrated into ash. Kikyō had armed herself with the dirt from Onigumo's cave, which absorbed the bandit's lust for her. This demon served as a preview of what would happen to Naraku himself if he attempted to harm Kikyō. Given that its brain and ribs were exposed, it was highly possible that this incarnation was not fully completed.
Musō was the reincarnated Onigumo, Naraku's human kernel. Naraku dumped him in order to get rid of his feelings toward Kikyō, which Naraku felt impeded him from defeating her. Due to Onigumo's disfigurement, Musō was born faceless. He attacked scores of men until he found a face that he thought was appropriate for himself. At first, he appeared to have no memory, either of his life as a person, or as the human heart of the half-demon, Naraku, who shut him off shortly after being born so as to avoid being overcome by Musō's human feelings. Musō recalled his memories during his fights with Inuyasha, and also remembered his attraction to Kikyō. Musō was ultimately reabsorbed by Naraku after a short time of independence. He demonstrated infuriatingly powerful regenerative abilities, able to piece himself back together even after taking the Wind Scar from Inuyasha. However, had Musō's heart been destroyed, not only his, but Naraku's downfalls would have been assured. He also demonstrated the ability to shapeshift new limbs, spikes, multiple legs and arms, claws, and a tail. Unlike Naraku (post-Hakurai-zen), Musō could not regenerate his clothing, which forced him to constantly murder others for new outfits. He named himself "Musō" because that was the name of the monk that he killed for his face.
The Infant was a child with the ability to force other people into doing his bidding by manipulating the corruption within their hearts. He used his powers to track down the whereabouts of a Shikon no Tama Shard. During his journey, the Infant was cut in half by a priest's spiritual powers. The left half (which remained as the Infant) was taken by Kanna, while the right one (which became Hakudōshi) was put in the charge of Kagura. He later was placed inside the haku (life force) puppet, Mōryōmaru, in a plan to make it so strong that it would be capable of overthrowing Naraku. In Chapter 446, however, Naraku managed to absorb Mōryōmaru, and with it, the Infant. In Chapter 448, Naraku disappeared along with the Infant, and by his next appearance in Chapter 452, had reabsorbed him.
Hakudōshi was an incarnation that was separated from the Infant. Taking the form of a young child, he was able to tame the horse demon, Entei. He continued the Infant's quest to get the last Shikon no Tama Shard. After Naraku was nearly destroyed at the Boundary of the Afterlife, Hakudōshi was charged with flushing out Kikyō. Tiring of being under Naraku's thumb, just as the Infant and Kagura were, Hakudōshi created Mōryōmaru to protect his other half. They plotted to replace Naraku and take the Shikon Jewel for themselves. However, Naraku destroyed his barrier and left him without any Saimyōshō, which allowed Hakudōshi to be killed by Miroku's Wind Tunnel.
Naraku's ninth and final incarnation that first appeared in Chapter 396, as well as the Fourth Episode of The Final Act. Byakuya, of the Dreams was created to replace Kagura, and served the purpose of tracking down Mōryōmaru while keeping tabs on the full-extent of Inuyasha's Tessaiga. Byakuya used origami as his primary weapon, while also having the ability to weave illusions and warp others to different dimensions to avoid interference. Like Hakudōshi, he was linked to Naraku, although on a higher scale. If Naraku lost an arm, Byakuya lost the same limb as well. He was killed when Inuyasha used the Cutting Meidō (one of the Tessaiga's abilities) on him after he attacked Kagome. Unlike his other siblings, Byakuya accepted his death.
A character exclusive to Secret of the Divine Jewel.