"Your talent is actually quite good. In time, you won't be any worse than Uchiha Fugaku. What you lack right now is time." Uchiha Yi wasn't flattering Uchiha Mikoto. She really was that talented.
Uchiha Mikoto had the potential to awaken the Mangekyō Sharingan, but doing so was agonizing, and Uchiha Yi didn't want her to endure that pain just yet.
Rather than wait for her Mangekyō Sharingan to awaken, she would be better off training her Lightning Release and Fire Release. Uchiha Yi was also teaching her Swordsmanship, and she was capable of learning all of it.
Once she mastered those skills, Uchiha Mikoto would become one of the Uchiha Clan's finest even without the Mangekyō Sharingan.
Uchiha Mikoto was talented to begin with, and she had Uchiha Yi at her side. There was no question that she had the resources she needed. Just look at Obito Uchiha: after awakening his Mangekyō Sharingan and receiving Madara Uchiha's teachings, he had swiftly become strong enough to fight Minato Namikaze.
What Uchiha Yi knew was no less than what Madara Uchiha knew. He might fall short of Madara in Battle Experience, but in Ninjutsu, Taijutsu, and Yin-Yang Release, he was already his equal. In raw strength, Uchiha Yi was even stronger.
Uchiha Yi was confident he could train Uchiha Mikoto to surpass Madara Uchiha. The only thing she lacked was a White Zetsu body and Hashirama Senju's Cells.
Uchiha Yi had no intention of getting Hashirama Senju's Cells for her, either. Why should he transplant Hashirama's cells into Uchiha Mikoto?
Why couldn't he transplant his own cells into her instead?
Weren't his cells superior to Hashirama Senju's in every way?
But how could Uchiha Mikoto integrate his cells?
All sorts of ideas ran through Uchiha Yi's mind. Why couldn't he have