Of course you hit Black Zetsu—what, go beat up Obito again?
Yorin had already cleared [Emergency Quest: Overawe the Uchiha] on Obito and happily claimed the +2 Stamina reward.
Given that, pounding Obito one more time would bring nothing but personal satisfaction.
Black Zetsu is different. What Yorin lacks now isn't gear or power—it's a bit of Yin–Yang Release.
Uchiha-ryū Sky-Sunder is peerless, but it's still a tad life-costly. Swap it for Truth-Seeking Orbs and you're set.
If not, a Tailed Beast Bomb isn't unacceptable either. As a bridge, Yorin had already decided to shamelessly learn Flying Thunder God and Rasengan from Minato.
All buddies here—no way Minato refuses such a small ask, right?
Bottom line: Truth-Seeking Orbs—the invincible technique—are Yin–Yang based. So the system's Black & White Zetsu reward? Yorin had to have it.
So he chose Zetsu and ignored Obito. Problem?
Obito can keep the others busy.
Whether it's Kakashi and Obito screaming "Naruto!" "Sasuké!" at each other, or Minato wracked with guilt for Konoha and itching to depose himself—either way, they've got more to do than Uchiha Yorin here.
If Obito wants a fated showdown, it should be with Kakashi, not me.
"Why are you all staring?"
Yorin saw no issue at all—didn't get why everyone looked so shocked. "I'm going to pound the Yin–Yang guy. Any questions?"
Obito: "Of course there are!" Since everyone had already recognized him, Obito didn't care about face anymore and shouted outright: "I am your opponent, damn you! Don't ignore me!"
There were a million ways to refuse Obito's challenge. In the end Yorin chose the one with the most flying comment spam:
"Why is a loser yapping? Kakashi—handle him."
Hearing his name, Kakashi tensed reflexively: "Yes, Yorin-sama!"
Obito: "AAAAAA—!!!"
A second later, Obito and Kakashi slammed together—kicking off the fated duel that satisfied Yorin.
The gap between them was wider than shinobi vs. ninken—but Kakashi didn't pull a "Bumpkin & Bumpkin" routine. He charged without hesitation.
Which infuriated Obito even more: "Damn you—looking down on me!"
He wanted to lunge at Yorin—but Kakashi already had him tied up.
Obito outclassed Kakashi by a head or two—but behind Kakashi stood Shisui and Minato, set to jump in. The moment things tilted, two Kage would step onto the board and make him eat dirt.
That forced Obito to leave brainspace watching Shisui and Minato—of ten parts strength, he could only bring seven. Add Yorin's earlier goading, and Obito's meager IQ dipped into the red. He looked fierce, but it was empty motion. From seven parts strength, lop off two. Now five parts—just enough to go fifty–fifty with Kakashi.
Yorin: "I didn't Edo Tensei Rin for your sake. You're welcome."
He tossed one last barb at Obito, then turned to the now somewhat dumbstruck Black & White Zetsu:
"So—I just cracked the Three-Tails. That's… inconvenient for you, right?"
White Zetsu the idiot opened his mouth to speak—but Black Zetsu clamped down on the tongue and vocal cords:
"Of course. For such a powerful Kage to be defeated by you—Uchiha Yorin—you are a dangerous man."
He couldn't tell if Yorin was hinting at something, but either way the answer was watertight—worthy of a thousand-year-old fox.
Yes—the Three-Tails being shattered was a problem. Luckily, a tailed beast's essence is a mass of chakra and never truly "dies," or Zetsu would be done.
…
Yorin: "So—Wood Release?" he continued. Zetsu had no desire to answer.
Zetsu: "Heh-heh."
Yorin: "Interesting. Wanna spar?"
Zetsu: "Heh-heh."
Right then—Yorin felt something wrong underfoot.
He leapt—and the ground where he'd stood erupted in thick, vine-like growths—lunging to grab him.
No small trick—two lazy slashes didn't cut them clean.
…
Zetsu's ceiling and floor are absurd. The weakest Zetsu is genin tier; the strongest White Zetsu can be Kage-tier. With Black Zetsu riding shotgun—Wood Release and hyper-mobility Mayfly—the package is not weak.
This time, to cover Obito, Black Zetsu brought the strongest body he had. Still—only so much.
If Chōjūrō can blow you up, there's not much left to say.
Combat isn't his charm—intel and setups are. That's what Yorin coveted.
Shame for him—Kaguya's so-called will was never going to defect.
Given that…
Do we cut him down?
Sever the plotline, leave Ōtsutsuki Kaguya to rot in the black room forever?
Yorin thought a few seconds—and decided: no.
Without other Ōtsutsuki threats, maybe. But now—
Zetsu, you don't want your hard-resurrected mom turned into God Tree fruit by somebody else and eaten, do you?
Kaguya—you don't want other Ōtsutsuki to come, beat you down, and stomp you flat, do you?
Dangerous—playing with tigers. But facing worse invaders—even Kaguya the shrew might be worth "partnering" for a minute.
So—come on, Zetsu. Call me Dad.
Zetsu: "???"
Zetsu: "What?"
…
Zetsu didn't understand why Yorin's eyes suddenly went funny. He couldn't place it—but it wasn't good.
He'd seen his share of crazy Uchiha in a thousand years. None as crazy as Yorin.
If he could choose, he'd never fight him—but he couldn't. If Obito didn't run, he couldn't run. Best he could do was stall—pray Obito came to his senses.
"—Madara-sama, the plan has failed! We must retreat!!"
Facing Yorin, he also shouted to Obito—time to go!
Obito's answer: "Shut up!!!"
Zetsu: "…"
Yorin: "It's exhausting, working with an idiot like that, isn't it."
Zetsu: "Ah—ah?"
He reflexively almost agreed—but froze as he spoke—Yorin—Uchiha Yorin! When—Shadow Clone? Body Flicker?
The tactic was simple—the speed wasn't. At this speed—he could treat Zetsu like a mob and mow him down.
"—Madara-sama!!"
One move—just one. Yorin's long blade took White Zetsu's head; the Raikiri along the edge carbonized every cell in a flash.
Black Zetsu had already shrieked and melted underground—couldn't win.
Keep this up and he'd die for real. So—run. As for Obito—good luck. If he wanted out, Kamui could blink him away. If he didn't, Zetsu couldn't change his mind.