This fic is my interpretation of why some of the recent manga events have been happening and how it all ties together. Crack theory (and somewhat confusing), but it makes a certain sense.
We're almost there, Minazuki. So very close now. Just a little bit more and everything that began all those years ago will finally have been resolved.
"Bankai - Minazuki."
Minazuki boiled with excitement as Retsu continued her spar with Zaraki. Soon, so very soon, she will be able to enter her true form. Her true shikai and bankai. She will be able to reunite with the half of herself that had been lost when Yachiru first crossed swords with the young boy many centuries ago.
Minazuki was no ordinary zanpakuto. She was an ancient entity who only came into the world in the presence of a particular type of spirit. Her fellow such entities Zangetsu and Engetsu only dwelled with those with a passion to protect so strong it overrode all other desires, morals, or values. She was not so noble; she sought only the truly bloodthirsty. Morals, values, passions were all frivolous ideas- nothing mattered to her but violence and an endless desire to fight and kill.
Yachiru was the first shinigami who fulfilled her requirements in billions of years. She, Zangetsu, and Engetsu had seen the spiritual realm and living world being destroyed and rebuilt many times over the course, and she had watch every time as Zangetsu and Engestu found and left her to be with who they deemed as their "partner" for the time being. She had waited alone for billions of years before Yachiru finally came about. They'd only been together for a couple thousand years, though, before meeting the young boy.
She'd been overjoyed. After waiting alone for billions of years, she'd found two partners within the span of a couple thousand years. The second the boy's sword pierced Yachiru, both of them knew that this boy was the new, true wielder of Minazuki. But what neither of them expected was the boy would close off halfway during the transferral process and leave it incomplete. For a transfer of Minazuki's partnership between two living souls, the current wielder must die by the hand of the new wielder. When that was complete, Minazuki could consume the blood of her dead wielder and manifest as the new wielder's zanpakuto partner. But the boy had stopped before Yachiru's death. Yachiru was no longer Yachiru, and Minazuki now a shadow of her true self. The fight in them both had been thoroughly transferred into the young boy, and as "Retsu" stood over his fallen form, she and Minazuki made a promise that someday, they'd finish what had already been started.
...but most importantly, you can finally...form a bond with your partner!
Zaraki Kenpachi.
At last, she could call him by that name. At last, everything was complete. She owed everything to Retsu - had the woman not perserved in living to bring the boy back all these years, she would've been stuck in her half-completed form for the rest of eternity. But now, the ritual was about to finally be completed despite the boy's - no, Zaraki Kenpachi's - desperate protests.
He didn't understand. He didn't understand that once complete, he would truly come to understand combat. He'd had a brief glimpse in the final spar with Retsu, but he'd yet to know how many times more intense "true battle" was when she, Minazuki, was part of it. He'd never figured her out or acknowledged her presence despite her calling to him all these years.
Katen Kyokotsu had. She had not expected an Asauchi-based zanpakuto to be able to discern her true nature. But somehow, Katen Kyokotsu had figured out not only that she, Zangetsu, and Engetsu were non-Asauchi zanpakuto spirits but also that she'd been in a dormant state. Not even Ryujin Jakka had been able to do so - he had not had a clue. His wielder had simply told Retsu to stay as she was in hopes of keeping her obedient under his command.
Perhaps the Vandenreich invasion had inspired Kyouraku to do otherwise - after all, the Quincy's "Bankai stealing" was done by emulating an Asauchi and transferring the imprinted spirit onto the medallion. It would only work if the Bankai was from an Asauchi spirit. She, Zangetsu, and Engetsu were different. They had nothing to do with Asauchi. They were not "under command" and could not be "controlled" by their wielders - they did as they liked. It was why none of them had a "release command", and also why Zangetsu was always running rampant with his current inexperienced partner. However, at the current time, it seemed both Zangetsu and Engetsu out of commission for a seemingly long time. Restoring her to full power was Soul Society's only chance for survival.
Not that she cared. She just wanted to finally be reunited with her other half and her true partner after having been isolated from battle for a few thousand years.
"I have watched over you all this time, Zaraki Kenpachi. I am..."