Kazane's first fever came only a few days later.
Her skin burned hot beneath Kushina's palm, though there was no infection, no illness the medic-nin could name. Chakra shimmered faintly around her crib, distorting the air in soft waves.
The fever did not break.
At first, the medic-nin assured them it would.
"She's adjusting," one said gently, though his hands trembled as he monitored the infant's pulse. "Some children with strong chakra networks experience instability."
Strong was an understatement.
By the end of the second day, the air in the room felt thick enough to swallow.
Kazane's tiny body lay rigid against the blankets, skin flushed, breath coming in shallow, uneven pulls. Faint distortions shimmered around her like heat rising from stone.
Every time she whimpered, the windows rattled.
They had the medic-nin check her condition again and again.
Each time, the result was the same.
No infection or diagnosable illness. No foreign chakra. Just her own chakra overloading her system.
So much chakra was overflowing from their child, Kushina, and Minato had to place seals all over their home to prevent it from spreading beyond the perimeter. The seals never lasted long, though, soon burnt out by the energy pouring out of Kazane. They couldn't let anyone find out because then rumours would spread. Spies would take notice, and their daughter would be placed in even greater danger.
Kushina refused to leave her side.
"She's burning up," she whispered hoarsely, pressing a cool cloth to Kazane's forehead. The water sizzled faintly when it touched her skin.
A medic-nin attempted a stabilising infusion technique.
The moment his chakra touched Kazane's.
It was thrown back. Not violently, just overwhelmed.
He stumbled, nearly falling. "I can't regulate it," he admitted, pale. "Her chakra isn't rejecting assistance, it's-"
What sounded like voices interrupted from outside. Before anyone could go to investigate, the door swung open.
Jiraiya entered first, travel-worn and unusually silent. Behind him was a woman with long tied-back blonde hair and green eyes. A silent pressure filled the room without trying as soon as she entered.
It took Jirya three days to locate Tsunade. Three days of Kazane drifting in and out of restless consciousness. Three long days of Kushina refusing to sleep... Three dread-filled days of Minato calculating possibilities he did not want to consider.
But find her he did.
Tsunade did not waste time on greetings.
"Move."
The word carried no cruelty, only urgency as she pushed past Jiraiya. Tsunade placed her hand lightly over Kazane's chest, contact lasting less than a second before her expression changed.
"Well," she muttered. "That's inconvenient."
Minato stepped forward. "What is it?" But Tsunade didn't answer.
She closed her eyes, letting her chakra slip into Kazane's system like silk through water. Unlike the other medical-nin, she did not attempt to contain the child's overwhelming chakra through force. Instead, she blended with it and observed the flow.
Minutes passed. The room remained utterly still.
Finally, Tsunade withdrew her hand.
"She's not sick," she said.
Kushina let out a strangled laugh. "We know that."
"Her chakra coils developed abnormally fast. Already, they resemble a fully grown shinobi's, they're producing and circulating chakra at a rate her body can't match."
"We have tried sealing, but it didn't work," said the medic-nin.
Tsunade's eyes snapped to his.
"You seal that down," she said coldly, "and you'll stunt her development permanently. You don't cork a volcano and expect the mountain to stay intact."
Silence.
Kushina swallowed. "Can you help her?"
Tsunade looked down at the tiny, overheated infant struggling to breathe.
"Teach her control," she said. "Immediately. Academy exercises. Not child's play."
"She's barely a week old," Minato said quietly.
"And she'll barely make it a year if you don't start now," Tsunade replied. "She doesn't need suppression. She needs regulation. She has to learn to distribute the load evenly and keep her chakra calm. Right now, every emotional fluctuation spikes her output, sending chakra tearing through her chakra pathways."
The words hit like a kunai.
Kushina's hands tightened around the blanket.
Jiraiya stepped closer. "And the Nine-Tails?"
Minato's expression darkened. "It's quiet."
Tsunade's gaze flicked to Kazane.
"When the seal weakened during birth," she said slowly, "some of its chakra might have mixed."
Kushina's breath hitched.
Minato's mind raced ahead, "The Nine-Tails' chakra catalysed her development?"
"Most likely, yes."
Again, Tsunade placed her palm gently against Kazane's abdomen.
"Fragile," she murmured. "Control won't be enough. Her body needs strengthening beyond what her monstrous chakra will provide."
"What do we do?" he asked.
Tsunade straightened.
"I'm staying," she said. "I'll start putting together a treatment plan. I suggest you start teaching her chakra control."
Silence followed that.
Kushina gawked incredulously, "You were serious?"
Tsunade's gaze sharpened. "I've never been more serious."
She folded her arms. "Chakra is the blending of physical and mental energy. If her reserves are that incredible, then both halves must be as well. Her body is struggling to contain it—but her mind?" Her eyes flicked to Kazane. "Her mind must be developing quickly as well, like many young geniuses."
Minato's expression was focused, analytical. "You think she can comprehend instructions?"
"Not words obviously," Tsunade said. "But actions? Patterns she can mimic, her condition undoubtedly gives her sensory abilities. She should be able to feel chakra and hopefully mimic that as well."
She stepped closer, placing two fingers lightly against Kazane's forehead. The air shimmered faintly in response.
"She's reacting to stimulus. Tracking fluctuations." Tsunade lowered her hand. "Like any infant, she will be curious."
Kushina's voice dropped. "So what are you saying?"
"Stimulate her senses," Tsunade replied bluntly. "Surround her with controlled chakra signatures. Repetition. Rhythm. Intent."
Minato absorbed that quickly. "Teach her to observe. To mirror."
"Exactly."
Kushina looked down at her daughter, brushing a thumb across Kazane's tiny knuckles. "She's just a baby."
"She's a baby with a chakra network denser than most chunin," Tsunade shot back. Then her tone softened—only slightly. "This isn't about turning her into a weapon. It's about her survival. If she learns instinctively—through observation, through mirroring—her condition will stabilise faster. Her body will eventually catch up with her condition, but until then, we have no other choice."
Minato nodded once. Decision made.
"What will you do?" he asked.
Tsunade turned toward the door. "I'm going to the hospital. I'll need access to archives, Senju research, Uzumaki records—anything on cellular reinforcement." Her eyes flicked toward Jiraiya. "And I'll need manpower."
"Manpower? You wound me, Tsunade."
Tsunade didn't roll her eyes—an indication of how grave this was.
"You're coming with me," she said. "If I'm drafting a regimen capable of reinforcing a body under constant internal chakra stress, I'll need a sealing consultant."
Jiraiya sobered fully at that. "Understood."
He glanced once at Kazane. The playful gleam in his eyes dimmed. "She's that bad?"
"She will be," Tsunade replied. "Unless we move quickly."
Kushina swallowed but held firm. "We'll handle the control."
Tsunade studied them both—Namikaze and jinchūriki, brilliance and power. Hopefully, the little one inherited both.
With that, she left. Jiraiya pushed off the wall to follow, sandals quiet against the polished floor.
Training began that night.
Minato exhaled slowly.
"All right," he murmured.
Kushina shifted, sitting cross-legged on the floor. Their daughter in her arms. Minato, opposite her.
Kazane blinked up at them.
Watching.
"Patterns," Minato said quietly.
Kushina nodded.
He released a thread of chakra—not a technique, not a flare. Just a thin, steady current. It flowed through his core in a controlled loop, smooth as a river in winter.
Kushina followed, hers warmer, fuller—but restrained. She adjusted to match his cadence.
Between them, a rhythm formed.
Kazane's chakra reacted immediately. It surged toward Minato's signature, then Kushina's.
"Match it. She needs something to sync to." Minato instructed.
The air trembled.
Minato looked down at his daughter.
He slowed his chakra deliberately.
Not weakening. Calming.
Kushina matched him.
Kazane's breathing hitched.
Her tiny fingers flexed.
Her chakra flared—
Then faltered.
Eyes met. Blue on blue.
For a fleeting moment, her chakra pulsed.
Intentionally.
Following the rhythm surrounding her.
Both felt it instantly.
"She's already learning," cried Kushina quietly.