In the last 3 months that Kai had been training with Ikkaku, he had unlocked 2 new abilities.
--1 month prior
Kai threw Shinten to the other side of the room with relative ease, as part of his white Shihakushō changed into a bloody red. He was launched upside down into the wall as he shouted,
"What the hell!? I thought that was just meant to be a light spar!?"
Even Ikkaku and Yuki were shocked at his sudden increase in strength. It wasn't that he had grown stronger, but his spiritual pressure had tripled to what it usually was. It had reached the level of a Lieutenant class Shinigami.
"Blue boy? What is that?", Ikkaku asked.
"I'm not sure either. Whenever I stimulate some of my spiritual pressure into my shorter Zanpakutō, I can feel something in there. I just tried to yank it out, and I started to be filled with this sudden strength."
He then felt a pain in his chest as the red began to fade into white, while his spiritual pressure decreased in conjunction. He fell to the floor, but Ikkaku quickly caught him.
"It was only this time, but I tried to yank out as much as I could? When I felt that mass of power, it felt like I could only draw out 5% of everything in there, maybe even less..."
"5%??"
5% seemed small on the surface, but a power jump that big defiantly left a large possibility on how strong Kai could get. If he could learn to draw it all out, his strength would certainty be amongst the higher tier of captains. Perhaps even above that.
He then noticed something strange about that power.
"That defiantly wasn't a Shikai or a Bankai release. It defiantly felt like he released a Zanpakutō, but the feeling it gave off was otherworldly."
That Zanpakutō was dangerous, both to everyone who fought against it, and to Kai himself. He would advise Kai not to use it often, and even then in small amounts. At first it was only 0.1%, but after training for a month he could use 3% for a maximum of 5 minutes.
Although he hadn't shown Ikkaku or anyone else, he had also achieved the release of Shikai.
--present day
Sakugo was very cautious of Kai's Shikai. It visibly hadn't changed at all from its sealed form, yet she felt cornered by it.
"What!? Were you bluffing?"
"I don't have to answer a dead persons questions."
That taunt was enough to mess with Sakugo. She immediately attacked in response to that remark.
"Go to hell!"
This time it wasn't just one, but several hundred bullet's of nitrogen that she fired at Kai. Not just in one direction, but from all sides, and even above his head, all raining down on him like heavy weather.
And in a single breath, all those attacks were pushed to ground. Small craters from the impacts of each of the bullets, the vibrations, could be seen in the water as they clashed against each other. Not a single one of them had even reached him.
The confused Sakugo was just about to increase the amount, and fire another volley, but Kai quickly struck first.
"Jigoku yōsai, 1%!"
Again. Another sharp increase in spiritual pressure. But only enough to push him into maybe the third seat.
He flash stepped in front of Sakugo who swapped to defence with her nitrogen barrier, but once she did, she felt that suffocating feeling once again, but this time it had grown even stronger.
Kai tried to pierce her right in the throat, but the barrier got in the way again. And much like before, it began to bend. Sakugo tried to leap back, but then she felt something stupidly heavy on top of her.
It pushed her into the water but she quickly tried to stand on it. The weight however was starting to make her tremble, and Kai was pushing further into the barrier, until it finally snapped.
Once it did, Kai felt a brief moment of peace.
But then Sakugo undid the spiritual pressure gathering under her feet, and let herself fall into the waters below, causing him to miss his thrust.
"Dammit."
He quickly was about to jump back, but then the water below his feet began to rise as it blasted him into the air, launching him into the sky.
"YOU MADE MY DRESS GET WET YOU BASTARD!"
She gathered up a large amount of the water just like before, and fired it all as a single compressed blast with enough pressure to perhaps slice up a building. But Kai simply pointed his sword at it, and it easily split in half.
"You have too many weapons in this place. We're taking this fight elsewhere."
He lifted his sword and pointed it towards the sky. As he did, Sakugo finally understood his Zanpakutō's ability. It wasn't just her, but the entire environment shifted in a single moment.
Everything began to levitate upwards. The water, the air, Sakugo herself, and even the Reishi in the surroundings. It wasn't that they were ascending upwards, but rather they were falling towards the sky while Kai remained as the only one unaffected.
Rather than experiencing the feeling of being upside down, Sakugo's senses felt as if she was falling too. That suffocating feeling, and the lack of Kai's Zanpakutō's appearance changing. It all finally clicked together.
"It's gravity isn't it? Your Zanpakutō fused with gravity. That suffocating feeling was the pressure of your Zanpakutō being right next to me?"
Kai pointed his Zanpakutō at her once again, but rather than moving in for a stab, he manipulated gravity to push her further into the sky compared to everything else.
"Left becomes right, up becomes down, and even existing power becomes strengthened where I will it. I'll play with your life, before I kill you. Just like what you did to me."
Sakugo tried to fly by controlling the air under her feet and even tried to strike back.
But the harder she tried, the more useless it seemed to be. The direction she tried to push the air in was constantly changing unwillingly. Kai's gravity was getting in the way.
Kai himself couldn't fly traditionally, but instead he reduced the effects of gravity on his own body, letting him not fly but float. He could fly with it if he practiced, but he didn't have much experience with his Shikai.
Sakugo continued to tumble into the air, before falling through the hole that she used to reach the seventh floor, and ended up returning to the sixth. Kai also followed her before returning gravity back to normal.
Without a moments hesitation, she used fullbring on the debris before throwing it all at Kai. But as they drew closer to him, it became much harder to push until they stopped entirely.
"What!?"
The rubble which Sakugo started to act a blind fold to Kai. She used this opportunity to maneuverer around Kai in hopes of finding an opening. Instead, all the rubble she threw which had been suspended in the air, began to be thrown back at her.
She easily defended against them, but the real surprise came when Kai who was on the defensive, finally fought back.
"Jigoku yōsai, 3%."
A burst of strength exploded in him as he flash stepped in front of Sakugo once more, before slashing her across the torso. Blood spewed out, and as it did, Kai smiled.
"So it was red. I want to see more."
Kai was just about to go in for another slash, but then he felt something pierce him in the stomach. He coughed up blood as he fell onto his back.
"You idiot. I can tell how amateurish you are with your Shikai, so you can't attack and defend with them at the same time. I was hoping that bullet would go through your skull, but I'm not that used to fighting with critical damage like that."
The blood which had spurted onto the floor, began to rise and flow back into her body. The damaged flesh forced itself together, before fusing without so much a scar being left behind. Perfect regeneration. She controlled her own body using fullbring.
Kai tried to get up, but the draw back from summoning the Jigoku Shihakushō, on top of his already damaged soul had struck him again.
"Again... AGAIN!"
Kai forced himself back up, but his legs were shaking and he was sure to collapse. The voice of Mitsuna Hoshi, began to echo in his mind much like before.
"Your being too impatient. Let's run away and recover our strength. If you use that Shihakushō one more time, it will start to leave permanent damage."
"No.. NO! I was sure of it. I was enjoying cutting her up. I gave into my desires, and now I could die instead. If I run here, everything I've fought for will be for nothing. I don't care if I can't be a soul Reaper anymore, I'll kill her here and now!"
Sakugo prepared fire what should've been the finishing attack, while Kai was about to summon the full might of the Jigoku Shihakushō. But before he even could, a red cero crashed into Sakugo.
She instinctively defended, while Kai turned to see what attacked her.
"What the heck? Is that a dog?"
It was a hollow which resembled a small dog, but it had the spiritual pressure of not just a hollow, but a soul reaper too. It was very confusing.
Once Sakugo cleared the dust with a wave of her hand, she appeared to recognise it.
"Its you! That stupid dog! Why are you here?"
Further in the shadows, someone with a powerful Quincy spiritual pressure was making his way towards the three. It's power was defiantly in the range of captain, and it appeared to be angry. This spiritual pressure too was something Sakugo recognised.
"What are you doing here!? Brother!?"