Izuru's Zanpakutō began to transform as it emitted power suitable for his status as a lieutenant. Its form changed into something much more suitable for his fighting style, a long sword however its shape was strange.
In the place of where the tip of katana should be, its blade bent into two 90 degree angles, creating a shape more like a hook. Oddly the outside of the Zanpakutō was clearly not sharpened, and the the true weapon was the inside.
"What kind of a Zanpakutō is that?"
"How about I show you instead?"
Kira extended his free hand out to Kai, and casted,
"Hadō #58: Tenran!"
The air began to sir into a wide hurricane which he blasted from his hands. Kai was surprised as he expected Kira to fight in close quarters right away, but he easily dodged the attack by moving to the left.
Izuru flash stepped above him, before attempting to strike him with his Zanpakutō. Kai easily blocked it with his Wakizashi in his left hand, but suddenly felt his centre of balance change.
Izuru noticed this, and quickly thrusted him in the chest. Since that side was dull, he wasn't cut. Kai quickly recovered and attempted to cut Kira with his Zanpakutō, but Kira jumped over it which surprised him.
He still should've been physically superior, yet something had changed.
Kira swiftly delivered another blow to Kai's chest before he landed behind him. Kai quickly tried to swing at him again, but Kira slashed his back twice before flash stepping away.
Kai was about to flash step to him, but for some reason he couldn't. The thing that had changed wasn't Izuru, but himself.
Izuru flash stepped in front of him again so Kai tried to pierce him, but it was easily dodged as Izuru slashed him several times on his arms and body, all with the blunt side of his sword.
Every time he did, Kai's movements slowed more and more, until the ground beneath his feet began to crack. All in all, Kai had been hit 7 times by that Zanpakutō.
Eventually Kai fell to his knees, while feeling a terrible weight crushing him. Izuru then walked behind Kai and put the sharp side of his blade below Kai's neck, preparing to decapacitate him.
"My Zanpakutō, it doubles the weight of whatever it hits. When I hit your swords, it's weight doubled so your centre of mass changed ever so slightly. Once I hit you in the body this fight was as good as over. Now then, any last words?"
Kai was quiet for a few moments, before suddenly laughing. Izuru wondered what was so funny about this situation as someone was going to die. But he gladly answered him.
"It's funny really. Ever since the day of the entrance exam I had felt your spiritual pressure all throughout my body. Seeing how powerful a Lieutenant was really had me wondering, was it really possible to beat someone like that? Nevertheless a captain? And then once I fought Akimitsu, I still wasn't sure if I could beat you... but now, I think I could even kill you."
Izuru suddenly jumped away as he felt something dangerous come from Kai. He emitted a wave of bloodlust that lasted only a single second, but it was enough for him to feel danger.
"Lieutenant Kira, I have to thank you. Maybe it's because our Zanpakutō's are similar, that fate had drawn them together. But thanks to this experience, the both of us have gotten stronger. Bury the light, Mitsuna Hoshi."
Kira was confused as his Zanpakutō hadn't changed, visibly anyway. But it didn't take long for it's effects to take place.
Kai quickly got back up, as if the weight that had been doubled never existed. And then suddenly he disappeared. This alarmed Izuru as he quickly tried to pin point Kai's spiritual pressure, but it was moving far too fast for him to lock onto properly.
At one moment he was behind him, and so he attacked with hesitation, yet there was nothing there.
Izuru fired a Shakkahō at another place he felt Kai's spiritual pressure, but it had long disappeared.
Kai's wakizashi then came flying at him out of nowhere, which Izuru quickly deflected, but then Kai appeared behind him
Izuru was surprised and tried to react quickly, but then he felt a blade tear through his torso. He was moving even faster than before he was even hit by Wabisuke.
Hisagi then asked Mayuri what was going on, and luckily the mad scientist had already figured it out.
"Everything in this world is influenced by gravity. From the movement of air molecules in the atmosphere, to the way the waves move by being influenced by the moon. This fact is also true for weight. I've already deduced that Mitsuna Hoshi is a Zanpakutō that controls gravity, and I suspect that whatever Kai did, had lessened the burden of the weight Wabisuke had doubled, and that somehow he reduced it even further, allowing him to move at speeds unbound by the laws of physics."
Mayuri was correct. Weight is just the mass of an object affected by gravity, and weight was the product of that change. So every time Kira doubled the weight of an object, he also doubled its mass, and in turn the formula for that object would change.
Kai knew this from a lesson he suddenly remembered from the world of the living, and in turn he knew that if gravity had been reduced on his body, the weight that Kira added to him would reduce.
But it didn't end there, he could take this even further. By decreasing the gravity on his body even further then what Kira had affected, his weight would further decrease to a level that affected his speed. By using spiritual pressure to provide a slight boost, he could go past the physical limits that the Jigoku had put on him.
Izuru quickly waved his sword at him, but Kai deflected it with his own. Since his swords weight had increased once again, Kai instead decided to let go of it. Everyone questioned why?
Rather than decreasing gravity, he increased it further, specifically in his right arm which he then smashed into Kira's rib cage.
The power from the punch sent him flying back into the wall, creating a massive crater which knocked him out. Kai's right arm spontaneously began to bleed, but he didn't complain from the pain.
"I swore I would protect that which is close to me, but I failed. I realize that I'm weak, and the cause of it was my desire to protect. It's because I chased a man whose dead, it's because I chased a stupid ambition that the people who I cared about in Soul Society are now gone. A stupid ambition like that... I don't need it. I swear by it, as long as my right eye is missing, I won't rest until that man is dead."
Seeing that Izuru had lost, Mayuri called him useless.
"Oh well, I guess that's that. Since he doesn't know anything about himself, I suppose I'll just sedate him and inspect his body."
Hisagi was about to stop Mayuri from pressing a button that would sedate everyone in the combat room, but then something suddenly began to ring in his pocket. He quickly pulled out and picked up the call. Once he finished conversing with the person on the other side, he let out a sigh of disappointment.
"Who was that?", Hisagi asked.
Mayuri's face gradually turned into frustration.
"I didn't think those people would set foot down here ever again, nevertheless this soon."