If Naruto represents the Sage then who represents the younger son?

[GhostProject]

Incorrect, Tobi (pretending to be Madara at that time) was merely lying to Naruto saying he represents the Younger Son, for a few dozen chapters later he said this:

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He referred to Naruto as merely a "pawn", the Senju vs Uchiha dichotomy represents equal opposing factions, but clearly Tobi/Obito did not consider Naruto as such. Ergo, Tobi/Obito was lying to Naruto when he said that.

And there is one vital element that the Younger Son/Senju lacks that Naruto, as an Uzumaki, has: friendship with the Bijuus the Bijuus have no love for either Senju or Uchiha.

It's plausible that Obito didn't believe what he was saying was the truth. It is possible though that it was simply his nihilistic approach to the Sons idea, that he noticed Naruto and Sasuke's similarities to Hashirama and Madara that established at the end of part 1, but his nihilistic views undermined their role to its most basic form, as simple stimulation for their fight.

On the Bijuu friendship, Rikudou's prophecy was that the one who would surpass him would lead the Bijuus down the correct path and show true power (love). The original younger son most likely lacked this connection and failed, so perhaps it is key to the success of the Younger Son, who will be able to express love as true power, where Rikudou admitted he could not in his life time.

Naruto has to be the Younger Son to represent his success as the true heir to Rikudou's work, and accomplish what Rikudou could not, through true power

I do however see what you are saying, and it is plausible that it swings either way.

 

[Honord Sage]

Madara First Son, First Hokage the second son, Naruto represents the original over all philosophy of the SO6P.