"I can change you back into a Nekomata."
Kuroka's mind blanked-out after hearing Kisuke's offer. Kisuke didn't say anything else and just enjoyed the hot spring while he waits for her to properly process what he means. Koneko was also dumbfounded as she didn't think it would be possible to go back after reincarnating into a Devil. Yoruichi and Aika stopped what they are doing because they don't want to interrupt what seems to be an important decision for Kuroka.
After a minute or two, Kuroka finally made a sound, "H-how...?" She didn't want to believe something preposterous could happen. As far as she knew, being reincarnated into a Devil forever changes your physique and won't go back even if you remove the Evil Piece embedded. The only way for you to change 'back' is discarding your current Devil body and move your soul to a new non-Devil body.
Creating a new body is already a very huge problem as only a few beings can do it flawlessly, but messing with a soul another thing entirely. Just touching it will risk it from being dispersed and never coming back. The only proven ways you can transfer your soul into another body or container is with the use of a certain Longinus or with the help of another being with a powerful soul that also resides on your body.
With all of this, Kuroka can't believe that he could casually utter those words but she also hopes that he's capable of it as being a Devil is some sort of trauma for her and wanted to get rid of this stigma. So instead of questioning its possibility, she asks him how it would work.
"Well, the whole process is still in the works so I can't do it now. But I should be able to complete it within a few weeks of time. I can make it shorter if I focus on it, but I still have other things to prioritize. As for how it'll go, I would first remove the influence of the Evil Piece from your body and soul and transfer your soul to another container. After that, I would 'fix' your body to how it was before you become a Devil and return your soul into it."
"Fixing the body... That's hard enough yet you sound like you can handle souls just fine. I can hardly believe your words. What gives you so much confidence?"
"I do not dare claim that I'm the best when it comes to souls but I'm confident with my skills. Look at that embarrassing glasses girl. What do you think of her?"
"Who's embarrassing!?"
"A normal human?"
"Why don't you try using those black flames of yours on her?"
"What!? The moment my Kasha touches her, her soul would be burned to oblivion!"
"Don't worry about it~. I'll take responsibility~."
Kuroka hesitated for a few seconds but still agreed to him. A small wheel surrounded by black flames materialize on top on her fingertips and it flew over to Aika.
"I don't have any say in this!?"
Once it hits Aika, she groaned... she tried to, but noticed that even though the black flames are burning him, she doesn't feel anything from it.
"What!? How is that..." Kuroka suddenly remembered back when Yoruichi also took it head-on and said that she chose the wrong target. She then turns her attention back to the grinning Kisuke and waited for his explanation.
"This Kasha of yours is a spiritual attack that damages the soul directly. It won't work on those people who have a more powerful soul than you. Aika-chan may seem a normal person, but her dormant soul power is large enough to offset your attack."
"What!? How is that possible!? A normal human has a more powerful soul than me!?" Kuroka exclaimed but thinking about it again, "You're the reason? You can manipulate souls like this!?"
"Yep~. Transferring souls are actually pretty easy for me. The only hard part is removing the soul from the body as the body has Mana or Ki that can resist it. The larger the Mana or Ki the harder it is. And fixing the body is actually the hardest part as I would still need few data regarding Devil physique and Nekomata physique."
"..."
"So what would it be?"
"That offer is everything that I would want aside from the safety of Shirone, but I'm still having a hard time trusting you. After all, you're very similar to those guys I loathe."
"True enough... Then I would have to resort to a more drastic measure~."
Kuroka obviously didn't like his words and prepared herself.
Kisuke stared at Koneko and Koneko also stared at his eyes, 'What are you planning now?'
"At least you believe in my expertise in the soul, right?"
"... I suppose."
"Then you do know that it's possible for me to plant something nasty directly to the soul."
It finally hit Kuroka what he's trying to say, "You bastard!!! Did you do something to Shirone!?" The seal on Kuroka has weakened sufficiently that she could now fight back but didn't do so in fear that he activates something that'll hurt Koneko.
"Yup~." I indeed planted something inside her. I need insurance after all~." 'Although it's not as you think. It's just some protection spell I conveniently placed.'
"You scoundrel!" Kuroka released Koneko and stood up revealing her voluptuous body.
Kisuke wasn't affected by this and replied, "I won't deny that. So? What's your decision?"
"Fine! I'll be your pawn! I don't care about your other offers! Just don't hurt Shirone!"
Kisuke clapped his hands, "Cool! Since you're still a Devil, we can create a binding contract. And with my expertise of the soul, I'll be able to make something that binds that too."
"I don't care! My first term is you're to protect Shirone from harm to the best of your capabilities! Second is you have to remove everything you planted on Shirone's soul. And lastly, you're not allowed to plant anything further on both Shirone and me!" Kuroka is very worried about her little sister that she didn't think of any loopholes that Kisuke can take advantage of in her terms. The only thing inside her mind is that remove the threat inside Koneko as fast as possible.
"Great. My turn then. First is that you're to all the information that you have. Second is that you've to gather information for me. The third is that you can't come near and reveal any association between you and everyone from Urahara Shop openly unless I agree to it. Fourth is that you have to take the same training as Koneko-chan. And lastly, you'll allow me to change you back into a Nekomata when I'm ready."
What Kisuke and Yoruichi want is just an eye in the darkness where they can't conveniently always come. The fourth term is for Koneko. Both of them didn't like that Koneko is virtually threading an unknown path alone, so they wanted her older sister to accompany her. As for the last one, Kisuke wanted to try it first on Kuroka before he proposes the same thing to Koneko in the future.