Kanna

Kanna (神無, kami mu) is the first detachment of Naraku. She appears to be a ten-year-old girl with pure white hair with white clothing. As a "concealed incarnation" of Naraku, Kanna has no scent and no demonic aura, making her undetectable to InuYasha's senses and is also immune to demonic aura-related effects such as the Hakurei-zan barrier. Kanna is the only incarnation that Naraku trusts with important information about his actions, allowing her to give orders in his place. Even though she is a void, she still has her own emotion of sadness that she cannot express.

Powers and Abilities
Kanna carries a mirror that can steal the souls of those reflected in it. Once a soul is trapped by her mirror, Kanna can control that person's body to carry out her bidding. However, the mirror can be overwhelmed by an especially powerful soul such as Kagome's. It can't steal souls from those who wear powerful items such as the Celestial Robe. The mirror can also create a glass demon to copy the abilities of whatever it reflects, though Kanna herself suffers damage when the glass demon is wounded. Also when Kanna has summoned the glass monster she can create a shadow of her mirror to shoot rays of light at to her opponent. She can even disappear and reappear at will indicating she also has Naraku's ability to teleport.

Biography
In the manga, Kanna first appears in volume 15, scroll 8 "The Mystery of the Wind Witch", page 152. There, Naraku tells Kagura "I have already sent your elder sister ... Kanna." Then Kanna is seen walking with her mirror and some Saimyōshō towards a village, where she will steal the souls of the inhabitants and turn them against InuYasha and his friends.

Tsubaki and Musou Arc
After her appearance she uses her mirror to spy on various things for Naraku while Kagura does the work.

Shichinintai Arc
She was ordered by Naraku to give orders to the Band of Seven.

Akago Arc
While the rest of her siblings chose to betray Naraku, she was the one who remained loyal to him. She felt sorry for Kagura because she had no real freedom. After Hakudoshi's and Kagura's death and Byakuya's creation, she was ordered by Naraku to pretend to help Moryomaru. Naraku knew Akago couldn't read her mind due to her status as Megami no Mu. As more and more of her siblings died, she started carrying more sadness in her heart. In episode 178 she went to the the flower field Kagura died in, finding her fan. It was at this point that Kanna started to question herself about whether Kagura got the freedom she wished for and realized that she herself had nothing.

Kanna's End
"Fools. Do they pity Kanna? But Kanna cannot feel a thing, be it pain, fear or sorrow. She cannot even understand the reason for your pity."

It becomes clear that she was fated to die by Naraku when she finally started to express her emotions (He reveals that her purpose for existence is to destroy them). In chapter 478 of InuYasha (Kanna's Gravestone), Kanna becomes more dangerous than anyone expected. She battles the group using a glass demon that came from her mirror. The glass demon copies the power of Tessaiga, robbing it of nearly all of its powers, including the Kaze no Kizu, Crimson Tessaiga, the Dragon-Scale power, and the Kongosōha, leaving the real Tessaiga nothing more than an apparent shell. It also appears that if the glass demon is hurt, the injuries are inflicted on Kanna since a black mark appeared on her neck after Kagome's arrow struck the glass demon in the same place with no apparent effect on it, and the same thing occurred when InuYasha struck the glass demon's left shoulder, which turns the demon into a fighting voodoo doll.

When the group finds Kanna, she fires a ray of light from the sky down at InuYasha. He reflects it with Tessaiga back at the glass demon. Kanna continues to receive damage to herself and the Tessaiga cracks from all of the light blasts. Kagome deduces that Kanna was ordered by Naraku to sacrifice her life in order to break Tessaiga. Because she continued to take most of the mirror demon's damage, Kanna indirectly loses her left hand and right arm during the battle. In chapter 480, she was forced to die at Naraku's command, by shattering herself and the glass demon into hundreds of shards.(In the anime, he destroys her heart.) A shard manages to hit Kagome's right eye and through the shard, Kanna communicates to her the secret to defeating Naraku.

Other than that, Kanna has never shown signs of betrayal to Naraku and was his most loyal, trustworthy detachment (besides Byakuya).

Naraku had commented upon Kanna's death that she had no thoughts and feelings; it was assumed that she did not care about either living or dying. However, after Kanna had communicated with Kagome just prior to her death, Kagome stated Kanna did not want to die and had feelings of her own. Kagome continues her wish to defeat Naraku and finally allows the Goddess of Void to rest in peace.

Kagura
Although never stated, it is possible that Kagura and Kanna are very close. As Kagura never had any ill thoughts about Kanna and Kagura and Kanna were almost always seen together in the Second Movie. It is also possible that Kagura's betrayal and death inspired Kanna to betray Naraku. Kagura's death may have also caused Kanna to question her existence.

Kagome
At first, Kagome and Kanna were simple enemies with one trying to destroy another. But as the series progressed and after Kagura died, Kagome became more worried about Kanna. As shown during Kanna's final battle, when Kanna was unleashed a mirror shadow on Inuyasha, Kagome warned Kanna about shooting the opening and when she died, Kagome screamed Kanna's name in clear disbelief. And after she died, Kanna spoke her last words to Kagome, telling her about the light that can kill Naraku.

Anime VS Manga
Kanna is given more characteristics in the anime than manga as the reason for her to betray Naraku was because of Kagura. It was revealed that she started to develop her own emotion over time and before she died, she smiled because she finally known of her own freedom.

Quotes

 * "Your soul is mine."
 * "Her soul...won't be contained."
 * "The longer I endure this world, the more beautiful the midnight moon."
 * "Flowers wither and lose their hue, much as I reflect in vain time lost to the long rain."
 * "Kagura, did you become the wind? Did you win your freedom?"
 * "Kagome..the light...the light will kill naraku."

Trivia

 * Kanna means 'god/soul of nothingness'