[Luminion Lancer]

Originally Posted by james0246

Spoiler for sorry, no real answer yet, but...:

Godd question. It is currently unknown how the Kyuubi si actually "split". Yes it is a yin and yang "split", but it is unknown what yin or yang mean in this context (chakra? mind? body? soul? etc). Suffice to say, the Kyuubi almost went all 9-tails during the Pain Invasion, so presumably all of the chakra is contained in Naruto...

Spoiler for manga:

-Hmm, Kishimoto did dodge to answer the whole question about Yin and Yang natured chakra so I'll bet that had something to do with it as well. Unlike elemental chakra, Yin and Yang seems like a special case. We can see that Naruto has enough chakra to almost complete a full Kyuubi so the split is not by quantity. What about malice? We have heard that the Kyuubi is summoned when enough human malice occurs. Perhaps Yondaime Hokage took with him the very source of that hatred, though the Kyuubi is still pretty pissed and malicious but then again, its a gigantic-monster-killer.