It is said that RSM is a gift from Hagoromo and it's resembled by the lack of pigmentation around his eyes while he is in Sage Mode:
The way he received it is because the Yang seal on his hand possesses Hagoromo's senjutsu chakra because Hagoromo possesses some kind of RSM without being Juubi's jinchuriki:
And when Naruto uses his KCM, his RSM is shown with it's full form and abilities like flying and and having TSB:
It's also stated in the Shikamaru novel that having the chakra of all the Bijuus is also a requirement.
So RSM requiars 2 thing:
1. Hagoromo's chakra
2. All 9 Bijuus chakra
When Hagoromo gave Lord Sasuke and Naruto the seals the chakra he gave was not permanent, because you need to give them body cells or Bijuu permanent chakra because Hagoromo was not immortal.
And Naruto is supposed to be the meeting place so he won't lose their chakra.
So when Naruto and Lord Sasuke fought they lost all the chakra they had from Hagoromo and the prove of this is this:
Adult Naruto does have pigmentation around his eye's while in sage mode and he only loses them when he is in his KCM(with 9 bijuu chakra):
And his eye's are orange not yellow like in RSM and his skin glows and doesn't have 9 tomoe on his back and can't fly or use TSB:
The reason that he has no pigmentation is because having all Bijuus chakra like with Juubi jinchurikis is also a way to attain RSM but it's really not enough to have the same effect Hagoromo's chakra had on him:
Some people think that Hagoromo's chakra is permanent because Lord Sasuke didn't lose his Rinnnegan.
The way Rinnegan is awakened is by merging Senju chakra with the Sharingan, and it didn't go back to EMS because Sasuke had Hachirama cells from Kabuto which means the chakra that comes from it is permenant:
Hagoromo only helped Sasuke awakening it by giving him already merged chakra which help the merging of the chakra he had before.
Do you agree?
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The way he received it is because the Yang seal on his hand possesses Hagoromo's senjutsu chakra because Hagoromo possesses some kind of RSM without being Juubi's jinchuriki:
You must be registered for see images
And when Naruto uses his KCM, his RSM is shown with it's full form and abilities like flying and and having TSB:
You must be registered for see images
It's also stated in the Shikamaru novel that having the chakra of all the Bijuus is also a requirement.
So RSM requiars 2 thing:
1. Hagoromo's chakra
2. All 9 Bijuus chakra
When Hagoromo gave Lord Sasuke and Naruto the seals the chakra he gave was not permanent, because you need to give them body cells or Bijuu permanent chakra because Hagoromo was not immortal.
And Naruto is supposed to be the meeting place so he won't lose their chakra.
So when Naruto and Lord Sasuke fought they lost all the chakra they had from Hagoromo and the prove of this is this:
You must be registered for see images
Adult Naruto does have pigmentation around his eye's while in sage mode and he only loses them when he is in his KCM(with 9 bijuu chakra):
And his eye's are orange not yellow like in RSM and his skin glows and doesn't have 9 tomoe on his back and can't fly or use TSB:
You must be registered for see images
The reason that he has no pigmentation is because having all Bijuus chakra like with Juubi jinchurikis is also a way to attain RSM but it's really not enough to have the same effect Hagoromo's chakra had on him:
You must be registered for see images
Some people think that Hagoromo's chakra is permanent because Lord Sasuke didn't lose his Rinnnegan.
The way Rinnegan is awakened is by merging Senju chakra with the Sharingan, and it didn't go back to EMS because Sasuke had Hachirama cells from Kabuto which means the chakra that comes from it is permenant:
You must be registered for see images
Hagoromo only helped Sasuke awakening it by giving him already merged chakra which help the merging of the chakra he had before.
Do you agree?