Mindscrapped

Among the three powerful souls, he could feel one that felt like complete and utter hatred, something that despised everything and anything without even needing a reason for it. Something's mere existence was enough to draw great anger from whoever the soul belonged to.

Ignoring that for now he moved onto the next, feeling something peculiar brush against his own soul... It felt like whatever it was wanted to grind him into ash, freeze it before melting it down and eating it, its goal? Complete annihilation.

Reima hadn't experienced what an unadulterated will to destroy had felt before, but now he had a front-row seat. It seemed to be trying to use some sort of magic to influence him, but it wasn't nearly strong enough to achieve anything of use.

Finally, he found the familiar aura of Diablo himself, even though his defences he could feel his subconscious fears rise up, prepared to force his hand in a sudden unwanted jolt. Of course, he was able to resist but everyone had fears, even a man with nothing to lose feared attachments so to reject what could very well make him human would be foolish idea.

Something to note was the fact that Diablo's soul seemed to be faintly linked with all the others through the weakest soul that belonged to Leah. It was originally her body and the souls were absorbed by her alone, forcing Diablo to interact with Leah herself to assume control. This had the obvious side effect of rendering Leah completely and utterly insane, or if not completely and mindlessly broken, as evil as the demons themselves.

One couldn't experience all the worlds evils simultaneously and come out as if nothing had happened, even Reima with all his defences, occlumency, soul strength, Haki, and mental fortitude wouldn't allow him to leave unscathed, let alone a simple human girl with no knowledge of the mind arts.

Reima didn't know if it was possible to restore her mind, but it wouldn't stop him from trying... His intentions weren't completely pure, primarily wishing to save the girl for Selina, her friends, and the knowledge that could be gleaned from trying to help her. Obviously, simply helping her was a motivator, but he'd seen enough of the world to know that helping everyone he came across was inefficient and would lead to people constantly asking for more, wishing to make gains from his sympathy.

He continued examining how Diablo's soul interacted with Leah's, his spiritual form frowning at how intricately they were linked together... It seemed that Leah wasn't as simple as he'd initially thought, the girl obviously sharing some prior link with Diablo himself.

This would likely be the most difficult thing he'd ever done when concerning souls, separating Akasha Bloodrivers soul from Alucard is like whistling compared to this... He'd literally need to unravel the connection between each of the weaker demons souls before threading it through Leah's, he couldn't cut the connection outright due to Leah's strange link to Diablo in the first place, which made the situation all the more difficult.

Regardless, he began the procedure anyway, not wishing to waste anymore time "inside another woman body", his girls would kick his teeth if they took it the wrong way... Fortunately, it didn't seem that time of the month yet.

Using Armament Haki to get a firm grip on the soul strands, he started separating Diablo's influence from the lesser demons. He also needed to keep his Conquerors Haki at full intensity to not only stop Diablo from interrupting him, but also the lesser demons from trying to escape and inadvertently tearing Leah's body apart.

He smirks to himself as the first one is untangled, instantly absorbing it into his chest where the dark sign used to rest. The souls power felt... About average? This shouldn't be misconstrued as weak, or even vaguely human though. Reima's "average" was about as strong as Ornsteins soul.

He continued, two souls, three souls, four... Diablo was struggling more fiercely now, the demon could feel his end slowly approaching. He knew that whatever Reima was doing wouldn't allow him to resurrect back in hell like how a demon usually would, his efforts were futile however as the more he tried to escape, the stronger Reima's Conquorerors Haki would be pressed.

Once he reached the two more powerful souls that were on a similar level to Diablo, he found that the restraints were like the others... Instead they were like steel cabals delicately wrapped around their souls to completely cover them. Not one inch of their wills to escape their prisons, and with their power being limited by Leah's soul, there was nothing they could do to escape.

It was difficult to unravel them, made even more so but their rabid attempts to strike out at Leah and their traitorous brother Diablo, but he subdued them and absorbed them like the others. Now, he was left with the hardest task out of the lot, separating Diablo's influence from Leah without destroying her soul.

Reima scowls as he feels Diablo's grip tighten on her and decides to try and loosen it in a more unconventional way... "Your penis is small Diablo." he projects this thought directly into the demons soul, getting a great deal of satisfaction by the colossal rage he received in response.

Diablo "I WILL DESTROY HER SOUL! LEAVE NOW!"

Reima "And I'll kick your ass again if you try, man you aren't even the strongest demon I've had to deal with. I mean, you can't even stop time!"

...

Reima gets the vague impression that Diablo doesn't believe him, this is good though, doubt is only the precursor to believing something.