After two days of research, Kisuke finally figured out what happened to Claire, or at least, a part of it.
It all had something to do with 'Faith' and Claire's connection to Heaven in the previous world.
In this world, the God of the Bible, Angels, Fallen Angels, and Devils don't exist. Even then, there's an existing faith.
Outside of unique abilities, one of the most powerful and mysterious moving forces is the congregated will of countless people that could even create a being that would later split up into the God of the Bible and the original Devil.
But for this congregated will to come to fruition, it needed substance.
Ki follows only the will of one. Magic Power is almost nonexistent in the plane. Lastly, Spiritual Power is solely managed by the Soul Society and any abnormalities in it are immediately corrected. Because of all of these factors, this 'Faith' couldn't be realized and remained trapped in this world with nowhere to go.
However, Claire amassing a vast amount of Holy Power on her own, triggered the homeless 'Faith' and it all cascaded into her as if finally finding their origin and their rightful place.
'Thankfully, it didn't create another being like the God of the Bible, but in exchange for that, Claire's physiology underwent a massive change to accommodate this 'Faith' that's now automatically converted into Holy Power, allowing her to grow Angel Wings and a Halo in accordance to the origin of Claire's Holy Power.'
Naturally, her Devil side wouldn't allow that change and fiercely fought back and that started the massive conflict inside her body that almost took away her life.
Thanks to the Reversal technique that Kisuke and Serafall learned from Grigori, they were able to balance things out since both of them felt it would be disastrous if either side won that contest.
As Claire managed to find a way to balance and stop these two sides from clashing head-on, Kisuke and Serafall immediately moved to stabilize Claire's condition, or more specifically, keep her alive as her whole body changed because of this 'Faith'.
Kisuke and Serafall didn't want to take chances so they asked Kunou and Tanis to help out which turned out to be a very great idea as the former was able to expel the excess Holy Power and the latter with her great healing prowess.
And after three days of nonstop care, Claire's transformation finished and the confirmation that the 'Faith' exploded its last brilliance of Holy Power before returning to nature and calming down.
'As expected... this 'Faith' became temporarily connected to the previous world, creating a pathway between the worlds. Although it's too small for it to be of any use right now, the only thing that could go through is this 'Faith' and nothing else. I wonder how they are taking it on that side?' Kisuke thought to himself while doing the final check on the ring that could restrict the sudden and unbalanced growth of both her Demonic Power and Holy Power.
Although the transformation was finished, Kisuke guessed that it's still just in its initial stages and Claire would have to get used to it if she wanted to grow normally, 'Whether that's a stunted growth or an accelerated one, I still don't have any idea. In any case, a half Angel and half Devil should provide more insights into the inner workings of these opposing but interchangeable attributes.'
Kisuke wondered if Demonic Power and Holy Power were at both ends of the spectrum, is it possible for something completely in the middle of it to exist on its own?
Kisuke sighed and kept the ring inside his Shihakushou, 'Now that all of that is out of the way, I have to deal with this troublesome situation.' He thought about how he should face an old friend who saw him as an enemy or a rival most of the time.
'There's no point in delaying the inevitable.'
Exiting his laboratory, Kisuke did a quick scan of the surroundings and found that everyone, except for Flanna, who was nowhere to be found, was in the living room, including their guest, 'How long have I been in my lab again? Two days? So Soifon-san should have fully expelled the miasma and recovered a considerable amount of her strength.'
Kisuke prepared to be attacked as he made his way to the living room.
But upon arrival, instead of the usual hostility, Kisuke could only feel awkwardness from Soifon which threw him off a bit, '...What the hell?'
Though before he could greet anyone, Claire came running and tackle hugged him, "Nii-chan!"
Seeing that, Kisuke's expression immediately softened and patted her head, "Don't push yourself too much. How are you?"
Claire rubbed her face on his chest and he could see some tears forming in the corner of her eyes, "I'm alright. Nii-chan... I'm sorry."
Kisuke was quite confused about why she was making a sad expression but finally understood that it was from guilt, "What are you talking about? None of that was your fault. In fact, it's me who should be sorry because I put you in that situation." Kisuke replied as he continued patting her head.
"No!" Claire looked up to him with her tears threatening to fall down, "It was I who became too reckless! It's not Nii-chan's fault!"
Kisuke chuckled and patted her head while also covering her eyes before looking towards Serafall, Tanis, and Kunou who shrugged their shoulders in confusion. 'So they also don't have any idea why she's acting like this. Must be some aftereffect of that transformation. I'll need to do another full checkup on her.'
Kisuke took his time to console the very sensitive Claire and was only able to walk forward when she went behind and climbed him like a koala refusing to leave its tree.
Kisuke ignored them and let Claire do whatever she wanted for now. And finally, he was able to greet Soifon, "It has been a while, Soifon-san. It's great to see you're doing alright."
However, Soifon wasn't able to reply and only looked at him strangely.
"Ah? Are you worried about my cousin? She's just a bit on the edge today but she should be alright."
Instead of returning the greeting, Soifon asked, "...Are you really the same Urahara Kisuke that I know?"
Since Kisuke couldn't sit down on the couch because that would squeeze Claire who was attached behind him, Kisuke took a stool out of his inventory and sat on it, "Am I really that different?"
"You feel like a completely different person to me."
Kisuke chuckled again, "And since you haven't started attacking me, you also feel quite a bit different from my memories. Are you really that hot-headed Soifon-san?"