Kira's in deeper trouble than her friends suspect. How do her ki-threads work and what are they made of?
Thank you, Ensatsu-Kokoryu-Ha, for all your encouragement.
I do not own any YYH characters. They belong to their author/creator Yoshihiro Togashi. The "Black Widow" name is also used by Marvel; I don't know anything about that character, so any similarities beyond name are completely coincidental.

Out of the corner of her eye, Kira saw Kurama's eyes flickering open. He was reviving. She spoke to him telepathically, telling him, "Run. Get help. Find Hiei." Akei said, "You're not going to run? Then why not attack? I love the sound of your threads snapping." Kira shuddered. "Stupid girl, I hurried through my last job because I was looking forward to destroying your soul." He leaned so close to her that she gagged at the foul heat of his breath. "In fact, I was so rushed that I didn't get a chance to eat any supper . . ." The transformation took only moments. Akei's fingers lengthened into claws, his teeth grew and became sharp, and his eyes turned a feral yellow. The bear-monster tackled Kira and opened its great jaws around her skull. Shutting her purple eyes, Kira fought the mindless terror that told her to lash out in any way possible.
Then the crushing weight was suddenly gone, along with the heat of the monster's breath. The creature let out a hideous scream as the rose whip inflicted another minor wound on its back. Kira heard Kurama's telepathic response, "I called you a friend earlier. I don't abandon my friends." The monster was on top of Kurama within a second, knocking the air from his lungs and sinking his teeth into the fox-demon's left arm. A second later, it was jerked into the air, where it hung suspended by almost-invisible threads. Kurama looked over at Kira where she knelt, surrounded by the purple aura of her power. He said, "I didn't think you had the power to use your ki-threads." She turned to look at him. Her forehead was covered with sweat, but her face was deathly pale in the light of the street lamps. He could hear her breathing heavily, and her eyes were dull. "I don't," she gasped, "I'm on borrowed time. Hurry and find Hiei." Kurama said, "I told you before. I don't abandon friends. Rose whip!" The whip flicked towards Akei, but became tangled in the ki-threads and didn't reach him. Kira, however, flinched as if she had been struck.
"Stop, Kurama, please! You'll break the threads . . ." She was trembling. "Please . . . Hiei . . ." Kurama frowned, but had no choice. Where could he look for Hiei, though? Clutching his bleeding left arm, Kurama ran.
Kira bent double and concentrated on continuing to breathe. She heard Akei's laughter, and looked up to see that he had transformed back to his human shape. "I wouldn't have believed it. Black Widow, selflessly sacrificing herself? The Black Widow who betrayed the one who raised her? Do you really expect Hiei to help you after you ratted him out to the Guild?" Akei tugged against the strings, and chuckled at her gasp of pain. "Don't faint yet. The real fun hasn't started." He moved one hammy hand to grab a couple of the threads.
Kira's mind was foggy with pain and weakness. She was shivering, and so were all of the threads. They were vibrating and creating beautiful, pure tones that harmonized perfectly . . . except that one tone was missing. When Akei grabbed a couple of the strings, the sound became discordant and harsh. She watched in a daze as the thread stretched thinner and thinner, and snapped.
The sound of the snapping thread seared into her mind and body. Her soul's scream woke every human psychic and rei-sensitive demon for miles.
Akei slapped her face lightly, bringing her back around. She noticed that he had freed himself, but carried the five unbroken threads in his left hand. He said, "I'll destroy the other pieces of your soul momentarily. But first, tell me why you are holding back. It isn't like you." He waited, watching her struggle for each breath. "Why didn't you attack with the seventh thread?" He waited again, but Kira didn't try to speak.
He pulled at another thread. Again, Kira heard the string's note rising as it was pulled tighter. The thread approached the breaking point.
Fire erupted around the two demons. In his surprise, Akei released the ki-threads. The shimmering strings melted back inside Kira's body. Hiei leapt between Kira and her attacker, his aura flaming red and yellow. His Jagan was open and glowing.
Someone grabbed Kira around the waist from behind. "It's me. Don't worry," Kurama said in his soft voice. "We'd better get back. Hiei's attacks can be explosive." "Hai," Kira agreed, allowing Kurama to half-carry her to the edge of the park. "How did you find him?" "Just after your psychic scream," the fox-demon said, "Hiei's aura flared. I haven't seen him this powered-up since he took the Black Dragon into his own body. I found him while he was running here." Kira relaxed back into Kurama's arms. She smelled the sharp tang of blood. "Kurama, your arm--" "--has already stopped bleeding," he assured her. "It's thanks to you that Akei didn't rip it off." Akei was backing away from Hiei, his eyes nervous but greedy. "Well, well, well. If it isn't the infamous master-assassin. Did you come to rescue your ex-partner?" Hiei stayed where he was, poised and still. "Why bother? She's the one who told the Guild the moment you escaped. She put every one of the Guild's assassins on your trail." Hiei snorted. "Baka. I didn't come to save her. I just don't like your smell. Get ready to die."