Uchiha Yin and Obito were little more than acquaintances. He couldn't follow Obito around all the time.
So if Black Zetsu and the others went after Obito as they had in the original story, all Yin could do was warn him and keep Rin alive.
If Rin survived, Obito probably wouldn't turn to darkness.
And if Obito didn't turn to darkness, Black Zetsu's plan would fail.
Yin was genuinely curious what Black Zetsu's face would look like then.
"Hello, Fifth Elder," Obito said, scratching his head with a foolish grin.
"Long time no see, Yin." Minato Namikaze gave him an apologetic smile and nodded.
"Long time no see, Minato." Yin smiled back in greeting. "What are you doing here?"
"We're on a D-Rank Mission, clearing out this stream. Are you training them?" Minato asked with a bright smile, glancing at the girls behind Yin.
The two groups looked each other over.
"Yin..." Kakashi studied him and the students behind him with a grave expression.
Clearly, Yin was training the girls.
After what had happened with Sara, Kakashi no longer dared underestimate Yin's teaching ability.
In just half a year, Yin had taken Sara from an ordinary person who knew nothing to somewhere between Genin and Chunin level.
Kakashi himself had become a Chunin years ago and was now working toward Special Jonin.
These students, though, had been in his class. They had only just graduated from the Ninja Academy and had barely begun training under Yin.
Even so, Kakashi couldn't dismiss them.
In fact, he was itching to fight one of Yin's students.
Kakashi was only nine now, yet he was already approaching Special Jonin strength—a full three years ahead of his original trajectory. (In the original story, Kakashi became a Jonin at twelve.)
That was partly because he had