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Kakashi's Onservation and Tactics
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In chapter 684, Kakashi notices that Kaguya totally disregards his group while she is focused on the main Naruto. Kakashi's Onservation and Tactics
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I'm thinking that, Kakashi devised a plan after observing Kaguya's ignorance and then told the group about it.
When Naruto and his clones were dealing with Kaguya and creating dust clouds (or snow clouds in this case), the real Naruto might've gone down to Kakashi's group and transformed himself into Kakashi while a few clones went to the other dimension.
Kaguya activated her Byakugan a bit later in the fight and Naruto barely escaped her sight. So she didn't catch Naruto transforming.
As for Kakashi, he probably blended in with the Naruto clone crowd or hid in the ice somewhere.
I also noticed that the Naruto that went to the other dimension was often referred to as the real Naruto. This might've been an exaggeration and Kaguya also believes it was the real Naruto that followed her.
In the last page, "Kakashi" mentions that he entered the other place safely. Which is actually maybe Naruto saying it since he transformed.
Their goal for now is to reunite with Sasuke and now that Kaguya is back in the Ice World, the Narutos and Kakashi could keep her occupied. Obito and Sakura would have some time to look for Sasuke. Oh yeah. And Naruto's there too probably if he wasn't completely destroyed or disappeared from Kaguya's stabbing.
Deception is crucial in a fight.
