Shin Uchiha thought it over and decided to tell Kakashi that his Sharingan was a Mangekyō Sharingan. Even if he said nothing, Kakashi would discover its secret in a few years, once he and the eye had grown sufficiently accustomed to each other.
He wanted to see what would happen if Kakashi discovered the eye's secret a few years early. He looked forward to seeing how the story would unfold afterward. He had already thrown this world into chaos, so one more change hardly mattered. Besides, he wanted to know whether Kakashi could drag Obito Uchiha out from behind the scenes ahead of time if he mastered the eye's dōjutsu-Kamui.
At that thought, Shin Uchiha felt eager to see what changes the future would bring.
Then Shin Uchiha spoke to Kakashi with a serious expression. "Kakashi, I'm sure you've noticed that you can't deactivate your Sharingan, probably because you don't have Uchiha blood. It's consuming your chakra every moment of every day. That indirectly slows your progress to a crawl and leaves you with limited chakra reserves. When you face a troublesome opponent, it puts you at a serious disadvantage."
After saying that, Shin Uchiha looked straight at Kakashi. "If you insist on keeping that Sharingan, it will have an enormous impact on your future. Your chances of reaching Kage level will be very slim. Even if you do reach it, you'll be among the weakest."
Kakashi closed his eyes in anguish at Shin Uchiha's words. Even though he had prepared himself, this harsh reality was still difficult to accept. After all, becoming strong was something everyone in the shinobi world yearned for and pursued. Now he had to choose: keep the memento Obito Uchiha had left him and let his strength stagnate, or give up the Sharingan, take up