Not sure if its been said dont flame me if its out there, this is just a theory.....
anyways i think that when whitey bit masaki, he attached a piece of his soul onto hers (like a horcrux...harry potter reference) mb its his special ability. the piece of his soul then laid dormant in her until ichigo was born and attached itself to him until it started to manifest itself..... I think that the reason ichigo has a zanpakto spirit but doesnt have an ausuchi is bc Zangetsu isnt a asuchi/zanpakto spirit. When whitey attached himself to masaki what if he also unknowingly attach a piece of the shinigami he once was. basically both zangetsu and whitey are the same guy and both inside ichigo doing their thing..
-zangetsus a high level shinigami
-aizen turns zangetsu into whitey
-whitey bites masaki
-whitey attaches piece of himself to masaki
-whitey/zangetsu are now inside masaki
-whitey/zangetsu move onto ichigo when hes born
-whitey/zangetsu manifest themselves inside ichigos sword
-ichigos now has an asuchi less sword
thanks for reading if you did.