On the night, when Naruto Uzumaki, after learning the forbidden A-rank Multi Shadow Clone Jutsu, defeated Mizuki and saved Iruka Umino, and he finally promoted to an official Genin.
On that very same night, a soul transmigrated from another world and merged with the soul of Konoha's Elite Jōnin, Kakashi Hatake. Inside a room of a medium-sized, Japanese-style wooden house, Kakashi was sitting on his bed, deeply focused on reading Jiraiya's Icha Icha Paradise book.
All the recent events, just two hours ago, involving Naruto, had given him a headache. After all, he was one of the few people who genuinely cared about Naruto, who was the son of his deceased sensei, and now after that event, he was relaxing before sleep.
Just then, another soul, from a different world, entered and merged with his. With a soft thud, he fell onto the bed he was sitting on. For the next few hours, he stayed unconscious as the two souls continued to merge. When he woke up, it was midnight in Konoha. He saw white moonlight shining through the room's window. Then he looked around him, not confused, but in disbelief.
He mumbled, "Is this all real? Or am I dreaming?" He firmly pinched his right cheek and groaned slightly from the small pain.
His one visible right eye widened in disbelief, and he let out a small laugh, "Haha, man, this is really true. I transmigrated, and that too into none other than Kakashi freaking Hatake!"
He laughed softly for a few minutes at this unbelievable thing, before he stopped, suddenly remembering something. "Damn, that means I actually died while working overtime, Fuck!"
He recalled his last moments before waking up as Kakashi. They were his usual routine of working non-stop to earn a little more money. He worked very hard in his last life, doing too much work, over-work to earn enough to fulfill his dreams, but in the end, he died at the young age of 26 due to poor health.
He then thought about his new reality, understanding that he had somehow transmigrated after his death and merged with Kakashi Hatake. Also, despite having both his own memories and Kakashi's, his own personality came first, rather Kakashi's, along with all the knowledge of his future, and this world's future.
He stood up from the bed and walked towards the mirror. He lifted the forehead protector from his left eye, where his Sharingan was. This time, both of his eyes widened.
He mumbled, "Wow, so my Three-Tomoe Sharingan, by itself, evolved to its true form, the Mangekyō Sharingan, the Kamui."
He thought for a moment and assumed, "Maybe it's because my soul merged with Kakashi's, and souls are related to spiritual energy, Yin Release. The Sharingan itself is the form of Yin Release. So that's why, with the increase of my spiritual energy, my Sharingan finally evolved to its true form, which it was supposed to have."
His brows furrowed as he thought, "But what's the point? My biggest problem is my chakra draining from the Sharingan. How could I even use the Mangekyō Sharingan when even the Three-Tomoe alone was the problem?"
With his hand under his chin, he walked back and sat on his bed, thinking and stated, "What will I do? I am Kakashi Hatake now, and this is my unchangeable, new reality. But now my Sharingan is little useless, because just a few uses of Kamui are enough to completely drain my chakra."
He then said, "Well, at least it's good that Obito wouldn't actually bother me even if he sensed me activating the Mangekyō Sharingan. Even in the anime, if Madara had never taken it from me, Kamui would have stayed in my left eye forever."
He lay down on the bed, looking at the ceiling, while covering his left eye again. He wished, "Huh, man, where is my transmigrated gift, my cheat, my system?"
Just as he spoke these words, he felt the air rippled in front of him. He narrowed his visible right eye as a transparent screen appeared, facing back at him. A small smirk tugged on his lips, hidden behind his face mask. He was delighted that at least he had a cheat of his own.
[Chakra System activated!]
[Remarks: This system helps Host increase his Chakra Reserves at a tremendous rate, with the potential to reach and even surpass Ten-Tail's reserves in quantity alone.]
[Choose one to activate the system in working:-]
[Choice 1: Trained and have your own fast Growth]
[Choice 2: Fixed increase rate Daily]
He carefully looked and analysis the system and guessed, "Ten-Tail's reserves in quantity? Maybe it means just in volume, but it wouldn't be the same quality as the God Tree Fruit, I suppose. But whatever, having a system is better than not, and this one sounds powerful."
He then looked at the two choices given to him and considered, "Hm, I've already done a lot of work. I don't want another bothersome, tiresome life. Besides, I'm already the infamous Copy Ninja, Kakashi Hatake. Apart from a few things, I don't need much training. So, I choose option 2, Fixed increase rate daily."
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