Normally, once a Sharingan leaves its original host's body, it becomes an independent entity. Even transplanted into another, without Uchiha blood, the eye cannot close nor evolve.
The Byakugan behaves similarly, but its chakra consumption is much lower than that of the Sharingan.
However, the Sharingan has a unique ability—visual synchronization. When the original owner experiences intense emotional trauma that awakens or evolves the Sharingan, the counterpart eye simultaneously transforms.
This is what happened with Obito and Kakashi.
The Sharingan awakens from deep emotional shock in Uchiha, producing chakra that manifests as tomoe and intricate patterns.
Kakashi, lacking Uchiha blood, cannot evolve his Sharingan naturally; he can only use and study it.
When Obito rushed to save Kakashi and Rin, his left eye caught sight of Rin's form, causing unbearable pain.
By then, the two Sharingan, now embedded in different people, had both evolved into Mangekyō Sharingan.
This was because the two Sharingan were originally one—through mutual resonance, they evolved together, each producing unique chakra that transformed their triple tomoe Sharingan into Mangekyō Sharingan!
"Do you see the boy's potential, Uchiha Fugaku?" someone asked.
"How is that possible..." Fugaku's eyes darkened as he stared at Yoru. "You're not Uchiha, yet you wield Uchiha secrets. This cannot be!"
He had sensed Shisui's potential when he took Shisui's eye.
He clearly felt that even with only three tomoe, Shisui's eye power rivaled his own Mangekyō Sharingan.
And this boy is only fourteen!
Without doubt, if Shisui had grown properly, he would awaken the Mangekyō Sharingan.
With this boy's eye, the Uchiha would have two pairs of Mangekyō Sharingan.
At that moment...
The legacy passed down by the clan's ancestors and the First Hokage for three generations would finally belong to them!
Before that happens...
He must preserve Shisui's talent—the future Mangekyō.
He must reclaim Shisui's eye!
Therefore, he followed Jiraiya to hunt Yoru.
Facing a youth like Shisui, Fugaku's heart was filled with dread.
If this boy matured, no one could predict the disaster he would bring to the Uchiha and Konoha.
To prevent that disaster, he must destroy Yoru while he is still young!
"I don't care where you got your information. If you're so stubborn, then die!" Fugaku lost patience and resolved to kill Yoru, even if he couldn't retrieve Shisui's eye.
His eyes narrowed, and Susanoo's massive fist crashed down with earth-shaking force!
Yoru dodged left and right, relying on Shunpo to evade the strikes.
But the Uchiha Flame Formation cramped the battlefield, limiting his movement and nearly landing multiple hits.
"Die!"
Fugaku's Mangekyō spun again. On Susanoo's skeletal frame, veins like muscle appeared, and two giant chakra swords materialized on its empty fists.
Yoru took a deep breath and dodged, landing steadily.
His right hand gripped the hilt of his shallow-cut blade tightly.
"I was saving points to buy a sacrificial body, but now…"
Yoru's gaze hardened, watching his score climb past 250,000 after defeating Hinata Hizashi (37,000), Yamanaka Haiichi (37,000), and Nara Suzaku (12,000).
"System, redeem Hyōrinmaru!"
[Redeeming 'Zanpakutō · Hyōrinmaru (Shikai)' requires 250,000 points. Redeem?]
[Redemption successful!]
[Detected 'Zanpakutō · Hyōrinmaru (Shikai)' and unnamed 'Shallow Cut'. Assign a name?]
"Name it!"
Without hesitation, Yoru's nearly depleted points surged with an icy chill flooding his body.
Suddenly, his vision darkened, then turned pale.
A howling cold wind blew, and the pale world before him shimmered with frost. Ice flowers blossomed, revealing an endless frozen landscape.
The biting wind swept through this world, and beneath the eternal icy sky, a terrifying presence silently gazed at Yoru.
Red eyes radiated solemn authority, a sinuous body forged of ice, spreading vast wings that stirred a blizzard.
"Hyōrinmaru," the icy dragon's voice echoed softly in Yoru's ear.
Suddenly, the snowy realm vanished, replaced by Fugaku's Susanoo, now evolved to the second stage.
"Have you given up resisting?"
Fugaku's eyes gleamed cold indifference.
Yet, the feeling was understandable.
Against the divine power of the Uchiha, no matter how gifted you are, what can you do?
Only despair.
Only powerlessness.
Regret crossing paths with the Uchiha, boy. This will be your eternal nightmare in the afterlife!
"It's over."
The cold voice was like the herald of death. Susanoo raised its giant sword and brought it down fiercely on Yoru.
At that moment, Yoru's eyes snapped open, icy frost surging upwards.
The clear sky darkened abruptly, heavy black clouds rolled in, and a torrential downpour began.
"Hyōrinmaru, awaken!"