Set when Isshin Shiba was the captain of the Tenth Squad and Matsumoto was the Lieutenant.
The Shinkai release of a sword is unique. Not in the sense that each Shinigami has a unique soul resting inside them, waiting to be manifested into a form. The experience of talking to a part of his soul varies from person to person. Take Hitsugaya Toshiro for example. With a powerful ice dragon for a Zanpakuto spirit, his inner world would be cold. According to him, talking to his spirit either allowed him to leave cooled and refreshed, or frozen to the bone in a very nasty frostbite-like way. Sort of like interacting with the man himself. You either leave happy after talking to such a proficient officer, or literally get a blast of his icy reiatsu for angering him in some way or another.
When Matsumoto was asked to describe her Jinzen experience, she would always make up a lie, especially when it was during training sessions with her squad soldiers. Well, who would admit that when they finally get into their soul world, their soul would usually be drunk off her ass, lying around and not doing anything productive? Well, Matsumoto was extremely tired of it. She wanted to boast that she was way better than her third seat, since her captain may or may not have hinted that his position may go to him if he achieved Bankai before her.
With that steadfast resolve to clean up her act, she took a week's leave and found a quiet cave to meditate. As soon as she got in her soul world, Matsumoto realized that her lazy, no good spirit, was once again hungover. It was lying and meowing on a huge pile of junk. Bottles of sake, magazines, papers and take out boxes just seem to be a few things that Matsumoto could identify from the junk pile Haineko was resting on.
"Haineko!" The irritated Shinigami called. She had a week to get as close to Bankai as she wanted (although no one had managed to achieve Bankai in such a short time.) Perhaps manifesting that good for nothing cat would be good enough.
The ash-grey cat snarled something along the lines of "Shut up" and "Go away". Matsumoto expected it alright, it was part of her as well, no? Drawing out a bottle of sake from her clothes, she offered the drink to the cat like a peace offering.
Surprisingly, it refused. Matsumoto was shocked to say the least. It had never ever refused a drink. The cat righted itself and began licking her paws, it's voice echoing through Matsumoto's mind. "Why have you come?" It asked. "If you are seeking to unlock my full power, don't even bother now. We can try it in another, say, 500 years time." On an ordinary day, she would have agreed, but Matsumoto was a girl on a mission and she wouldn't let a lazy cat get in her way.
"Yeah, I have come for Bankai. Just tell me your real name and we can make everyone's life easier." She started, knowing fully well Haineko would never concede so easily. No harm in asking. With a lazy yawn, the great cat rolled over on its back, seemingly contemplating her offer.
All most immediately the cat said, "Normally I would have to manifest in your good for nothing world, but since I'm hungover as hell, go and get me some sake. But not just sake, but enough sake to ensure that every Shinigami in the Gotei 13 would get drunk. If you buy me a bottle more or a bottle less, you'd be paying in blood." The cat told her, licking its paws, thinking about her nefarious plan to work her Shinigami to the bone.
Matsumoto nearly screamed. Calculating how much sake it would take to get someone drunk was a long process. Doing it for herself took a few days. Imagine the whole Gotei 13! All the data she would have to collect. Oh she felt tired already. So so tired. But her resolve for Bankai was stronger. Releasing herself from Jinzen, she got to work.
Day melted into night, and night melted back into day. The remaining days were spent counting each and every drop of sake. With some eyelash batting and bust showing, she managed to get her hands on Squad 12 equipment that could do the math for her in a few short hours. The rest of the time, however, she spent raiding every bar in Soul Society. The grand total for the sake was about 5 years of salary- a small price to pay for Bankai, she reasoned. Getting Bankai then being promoted to captain would ensure a high pay. So no harm done.
One day before her week was up, she had managed to collate enough sake for her Zanpakuto. Entering her soul world again, the cat smiled to her, saying the amount was just right. It did not cast more than a glance at the huge pile of sake. Matsumoto beamed. That wasn't so hard. A lot of heavy lifting and Shunpoing about might have killed her muscles, and her head nearly burst trying to figure out that machine for Squad 12. Well, it was well in the name of Bankai.
Yet, Haineko had other ideas. The second part of her task was to deliver the sake to every officer, and ensure they consume it within 12 hours. She nearly backed out. No way! She would be roasted by all the captains. A look from Haineko was all it took get her moving. Magically, (but really it was kido infused Hell butterflies) the sake was delivered. Save for some very uptight officers, it had been consumed too.
God was she tired. She had never worked so hard in her life. Her muscles were screaming, her head was pounding, and there was no way she would drag herself a few hundred meters to buy some sake to soothe her ache. Despite the throbbing pain everywhere, she once again entered her soul world.
Haineko was still resting on the junk. When its Shinigami approached, weary with all the work she had to do, it sniggered to itself. It sure was fun duping that airhead, it thought. Like a cat with a mouse, it just had to play some more. It beckoned for Matsumoto to come closer, and she complied, eagerly waiting for the name of the final release.
"Before I give you my true name, you have to prove yourself once more. Go and seduce the Soutaicho then I will truly be able to allow myself to be wielded by you." It said regally, although it was inches away from cackling in glee.
Matsumoto made a face. Soutaicho? He's old, wrinkly and just the thought of it. Eww. The cat almost was hoping that she couldn't do it. It wanted to see her fail, so it will never need to work again. She seethed. Well she'd show it what Matsumoto Rangiku could do!
With a fat humph, she left the soul world, showing off even more of her cleavage as she stormed to the First Division's offices, determined to master the cat once and for all. There was a few hours left till her leave was up, and she wanted to come back victorious. If her third seat could graduate from the Academy in a year, she can achieve Bankai in a week.
Hitsugaya had though he had seen many strange things in the land of the dead. As such, he didn't react when nearly all the captains stormed into the 10th Division offices the same day Matsumoto returned from her leave, demanding the Lieutenant be sacked. Nor when Matsumoto screamed at her sword, saying it was a lying cheating bastard in addition to being lazy ass good for nothing toothpick. When Matsumoto took the offending sword out and blasted it with Kido spell after Kido spell, then proceed to pour cold water on it, he reasoned that it was just the stress of being a Lieutenant, and let her be.
But what made his skin crawl with distaste and his mind churn, was that in the wee hours just before Matsumoto returned, the temperature in Soul Society sky rocketed. The third seat was sleeping peacefully, enjoying the chill of Autumn's night air, when it suddenly turned superbly hot and stuffy, so much so that he had trouble breathing. His head was spinning and beads of sweat dripped down his hair. Only one person could change the temperature like that in an instant. With an annoyed scowl, he released his Shinkai and encased his room in ice, cursing the person who pissed the Soutaicho off so much.
Matsumoto walked into the office at 8 am, resembling a piece of burnt wood. 12 hours later, she left the office resembling a popsicle, vowing to herself to never listen to her Zanpakuto again.