Let's see, speculation. Isshin is his father, Masaki is his mother, Masaki is a daughter of a hollow and a quincy. Everyone's happy. Which explains his Getsuga Tenshou works actually like a quincy arrow and why his sword can't be restored by a shinigami, it's not a sword, just looks like one. His Vasto Lorde powers are suppressed because the quincy-hollow experiment wasn't complete, but now he's some kind of monster with all the possible powers in the word.
Good job, Kubo, ruining everything to make the main char the strongest so he doesn't have any problems facing anyone whatsoever.