The mystery surrounding Hiei and Kira's shared past are being slowly revealed, and a new mystery about Kira's powers is hinted at.
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.

"I don't see what the big deal is," Yusuke complained. "It's just a couple of scratches." Kira snorted. "That's what they all say, until the nausea and vertigo hit you. The fever's not very pleasant, either." Kuwabara whistled. "The lizards were that poisonous? Why didn't you warn us?" "It's a mild poison, but Urameshi has at least twelve bites. How are you and Kurama?" Kurama said, "Neither of us were bitten. The lizards ignored us and swarmed over Yusuke." Back at Urameshi's apartment, Kurama made him drink a foul-smelling potion and go to bed. He explained, "I can counteract the fever, but we mostly need to wait for the poison to run its course. Yusuke should be fine in 36 hours." "Hn," Hiei muttered, "He shouldn't have let himself get bitten in the first place." The short demon opened the 14th story window and jumped to the sill.
"You've decided to let me live?" Kira leaned casually against a door frame, watching her former teacher closely. Hiei didn't answer. Kira asked, "Goodness, Hiei, you're not going soft, are you?" Hiei's glare ignited the wallpaper next to Kira's head, but she patted out the flame without taking her eyes off him. "Traitors like you aren't generally worth the effort to kill," Hiei growled. "But if you survive your attempt to baby-sit Yusuke, I'll make an exception." He leapt from the sill and disappeared.
Kurama asked, "Traitor?" Kira grinned. "He and I were partners. Both of us took care of ourselves and didn't care much if the other got screwed." Holding her sheathed sword like a teddy bear, she curled up on the couch. "I'm going to sleep. Wake me if any assassins come looking for me." Kurama and Kuwabara also stayed with Yusuke while he was feeling unwell. Kuwabara was snoring loudly on the floor of Urameshi's room. Kurama spread out his books on the coffee table in the living room and did his homework. Late in the evening, he realized that Kira was watching him. He said, "I didn't realize you were awake." Kira ignored that and asked, "Why are you doing your homework? Why do you bother?" Kurama shrugged. "It makes my mother happy." At the startled look on Kira's face, he explained, "I was badly wounded fourteen years ago. To survive, I took the form of a human fetus in a human woman. I refer to this woman as my mother." There was a long silence. "Isn't there someone you care about?" Kira seemed not to have heard the question. She mused quietly, "I've never seen this side of Hiei before." "Yes, he does have quite a temper." Kira blinked, surprised. "I wasn't talking about his temper. I meant that I've never seen Hiei so unguarded. He must really trust the three of you." Kurama's chin almost hit the table. Hiei unguarded? Hiei trusting? What was she talking about? Kira stared out the window Hiei had gone through. After several minutes, she walked to the door.
"Going somewhere?" Kurama asked.
"You going to stop me again?" "If you don't rest, you won't recover. Where are you going?" Kira flashed him an angry look. "None of your damn business." Kurama narrowed his eyes. "What's wrong? What aren't you telling us?" For a moment it looked as if she wasn't going to answer, but she finally said, "Do you remember what the old man said? The Assassin's Guild has sent Akei for me. Akei is . . . strong. Very strong." "You're scared?" Kurama was prepared for a violent denial, but Kira just nodded slowly. Surprised, he offered, "I'll come with you. I'm getting cramped from all this studying." Kira looked up into his friendly green eyes. "You aren't strong enough to beat Akei. Nor is the orange-haired idiot. And Urameshi's too sick." Kurama stiffened slightly at this affront to his pride, but said simply, "Then let's hope he doesn't attack until Yusuke's recovered. Come with me; there's a nice park a couple of blocks from here." Kira hesitated. "Fine. Have it your way," she said in a cool voice.
The two didn't talk on the walk to the park. Kira refused to tuck her sword and sheath into her belt, but carried it in her left hand. Kurama hoped they wouldn't get stopped by the police. When they reached the park, he leaned against a tree while Kira prowled the perimeter restlessly.
"Why don't you relax?" Kira glared at the red-haired fox demon. "Have you ever watched a demon be disemboweled and eaten by a wild animal while he's still alive?" Kurama blinked. "You don't understand. This demon almost destroyed me when I had all of my powers." "You fought?" "The Guild needs to be able to rank its members. There are few assassins I haven't fought. A couple of years ago, I challenged a high-ranked assassin to a death-duel." "Akei?" "No, this demon was only high C-class. I killed him and took his rank. Then Akei challenged me, crushed my technique and came close to destroying me completely. He snapped three of the seven primary threads." Kurama furrowed his brow. "You're talking about ki-threads? I thought they were unbreakable." "That's why Akei's so dangerous. My attack used the seven threads nearly simultaneously, but his speed is such that he was able to snap three of them before the others trapped him. He also had the strength to break them." "Why didn't you just make more? Yusuke said you attacked with hundreds of strings." "It's impossible to make more than seven. I doubled back the threads when I attacked Yusuke. I doubled them back when I attacked Akei as well, but it made no difference. He's too strong." A pleasant bass answered, "No, you are simply too weak." Kira whirled around and drew her sword; Kurama grabbed the rose he always kept in his hair. Akei stepped out of the bushes. He was a large demon, fully seven feet tall with broad shoulders and hands the size of dinner plates. The man smiled broadly, showing all of his teeth. "Do you really think that a sword will help you? You're too scared to move; I can smell your fear from here." Kira braced herself for an attack. Kurama transformed the rose into his signature rose whip and flicked it at Akei. The larger demon barely moved, but somehow dodged the stroke-- and the next one, and the next. Again without seeming to move, Akei suddenly appeared less than a foot in front of Kurama and grabbed the fox demon by the throat, lifting him easily into the air.
Instantly, the grass at Akei's feet came alive and grabbed at him. The large demon lifted his foot slightly, uprooting the plants without any effort at all. He batted the rose whip out of Kurama's hand. As black spots crept into the edge of his vision, Kurama desperately summoned the rose petals to attack. Akei simply swung the fox demon around, using him as a shield. The concussive force of being thrown to the ground knocked the rest of the fight out of Kurama, who drifted in and out of consciousness.
Akei looked at Kira and smiled again. She raised her sword, but it was already too late. He was in front of her before she could react. She swung, but he caught the sword by the blade and twisted it out of her grasp. He tossed the weapon aside. He asked, "Aren't you going to run? It's so much more fun that way."