User blog comment:Xilinoc/Ch. 540 - The Sword Five/@comment-4026406-20130605202342

I'm thinking that Ywach, when he was young, placed bits of his soul into his followers, creating the Quincy race, which was then passed down through families and clans. Those "bits" of his soul were preserved in pure blood quincies, but were diluted when normal humans intermingled. It would explain why the Quincy have such a stance on pure-blood "politics" and why he would want to "purify" the blood of the Quincies using his Selection.

Essentially, Ichigo's hidden shinigami nature allowed the bits of Ywach's soul to manifest quite literally into a spiritual form, and because he was younger when he put his soul into his followers, Ywach!Zangetsu looks like he was 1000 years ago.

The spiritual form of his Quincy powers eventually dominated the inside of Ichigo's soul, becoming the source of his powers. We do not know if Ywach!Zangetsu has malicious intent, or any kind of intent at all, but it's obvious that the blade that Ichigo once wielded was merely a Quincy spirit weapon, hence why Ouetsu said he would have to say goodbye to Zangetsu because it wasn't a true zanpakutou, more like a pseudo zanpakutou due to his hidden Shinigami nature.

Ywach!Zangetsu suppressed Ichigo's true shinigami powers, the hollow "White" (Hichigo) within because it was created from dead shinigami by Aizen, and pretended to be his zanpakutou, while keeping White at bay.

I'm thinking that Tensa Zangetsu's true form is the forced merging of Ywach!Zangetsu and White. I don't know if this was Zangetsu taking over White or if it was a mutual thing in order to keep Ichigo alive, but it's pretty obvious that even Tensa Zangetsu was an abomination. It does explain why Ichigo could have a Bankai, despite the fact that his original blade that was in constant Shikai never had a release state. His original blade did not have any true shinigami bits, but when he went into Bankai, Ywach!Zangetsu was stealing some powers from Hichigo.