Heart
Naraku could form a face easily enough. He could make hair, and limbs, and muscles and skin and cruelty and anger, but the part he had the most trouble with, the part he himself least understood, was the heart.
Kanna, his first venture into the art of making, was acardiac. No heart. No blood. Void resided in her arteries, and gave her power. An elegant solution to Naraku's shortcomings. She was never intended to have a heart, and her life force was tied into his own. That was why loyalty to him was a priority in her design, because she was the one person he could not kill.
But cold and dark and void are not things. They are absences, and they yearn to equalize. Kanna wanted a heart for no other reason than that she didn't have one, and she felt it, like cold things soaking up any available heat. Not a true emotion, but the nature of the vacuum.
When she first saw Kagura's heart, she assumed that it had been made for her. Entranced, she watched it beat bloodlessly, and stroked a pale finger along the supple musculature. Naraku had to keep it on his highest shelf from then onward, out of her reach.
Reaching was something Kanna wasn't good at. She would kneel beneath the shelf and listen to it beat, like a motherless puppy with a clock.
But it was not for her.
When the heart was used for Naraku's true purpose, Kanna felt the void. Silence is the absence of sound, and yearns for sound. She missed it with a lack of emotion that itself yearned for emotion.
Death is the absence of life, and Kanna yearned for life the way that only those who have never had it can. In death, deathless, she was immortal, and as long as Naraku lived could never know what it was to embrace true life and true death.
Kagura's fate did not frighten her, as one without life cannot perceive death as bad. Rather, she admired Kagura's way of both living and dying, as she had admired her heart so long ago.
Betrayal of Naraku was unthinkable to her, and yet...
Kanna longed to be warm, to flow with blood and to decay.