Satsuki leaned back as she gathered her thoughts, wondering how best to explain the situation with the Life Fibers now that it was resolved. While her audience was apparently a Devil and thus likely could suspend disbelief much further than an ordinary human, dropping too much on her too fast - as much fun as Ryuko was having with it - would likely cause her to either balk or break at this point. She seemed close to it already.
As her eyes briefly shifted to her former Elite Four, her closest friends, she realized the simplest way to start the explanation would be with the very same explanation she gave them. Turning back to Rias, she spoke. "Why do humans wear clothing?" she began calmly.
Rias opened her mouth to respond, then paused. "I...I want to say it's a result of coming to an understanding of modesty due to eating of the Tree of Knowledge...but that would imply modesty was a concept known to those sapient and sentient races older than Humanity...and yet artwork of the war between Angels and Devils of that era shows them naked, with no concept thereof. That implies the concept of modesty originated in humans, and the other races learned it from them to better infiltrate human society for their own ends...eventually making it a natural part of themselves. But...why would modesty happen in the first place?"
Satsuki smiled. It was good to see that Rias was rethinking things she'd 'known' for so long based on what she'd encountered now. It showed she was flexible and adaptable both physically and mentally. She might just come out the other side of knowing her family as something other than a gibbering mess. "That is where Life Fibers come in," Satsuki explained as Shiro handed her drawings for visual aids, the first of Life Fibers in their natural state. "They are extra-terrestrial parasites that feed on the neural energy of other forms of life. However, there are no life forms that can naturally survive having the Fibers inside them. Instead, the Fibers feed by wrapping around other living organisms." She accepted the next picture, this one of the classic 'evolution of man'. "They locate the species on the planet they arrive at with the most developed brain and nervous system and nudge it into further development, while implanting the concept of modesty and shame in their culture."
The next image showed a development of clothing, from leaves to furs to fabrics. "This results in that species making clothing, which the Life Fibers infuse, allowing them to feed off the neural energy in their dormant state. Thus they remain for millennia until they have spread to cover the majority of the species and they have fed on enough energy. Then they awaken."
The next image was an actual picture of humans being taken up into the sky to form the crimson Life Fiber cocoon around the planet. "Thus begins the Life Fiber Nativity, where the species is absorbed by the Life Fibers and surrounds the planet, becoming a cocoon to create more Life Fibers. When all life within the cocoon is Life Fibers..." The last image was of a planet exploding, crimson threads launching out into space. "The planet is caused to explode, the force scattering the Life Fibers into the cosmos to seed further planets."
She set the visual aids aside. "That is what Life Fibers are. Any questions?"
Rias stared at her, her jaw somewhat slack. "It...that's what the Crimson Night was? That's why no one in authority could figure out what caused it? Because there was no magic involved?" She frowned thoughtfully. "But...why didn't it affect us? Or anyone in this area?"
"No Life Fiber energy," Ryuko spoke up calmly. "When you gave me that out of body experience, I was able to see that you and Akeno had never been exposed to Life Fibers - and thus their energy - before meeting me. There isn't a trace of Life Fibers anywhere in your clothing."
"I would imagine that clothing that comes from Hell would not have Life Fibers in it," Satsuki concluded. "Tell me, is there a strong enough Devil presence and control at the school that the uniforms can come from Hell and not be noticed?"
Rias blinked in surprise. "Y...yes. The Headmaster is a Devil, as is the President of the Student Council. In fact, the rest of the Student Council is her peerage." She rubbed her chin thoughtfully. "So, anyone who wasn't exposed to Life Fibers would not have been taken up by the...Nativity. Would that just be anyone wearing Devil made clothing?"
"I wager anyone with strong ties to the occult would also be exempt," Satsuki concluded thoughtfully. "As magic and Life Fibers aren't naturally compatible, anyone with a strong enough magical presence would be most uncomfortable in clothing that had Life Fibers in it, and would seek alternatives."
Rias nodded thoughtfully. "I...suppose that makes sense. But how does that explain Ryuko?"
Satsuki willed her face not to harden, but from the way Rias flinched back, she knew the Devil had caught her momentary lapse in discipline in reaction to that inquiry. "I...will not go into details at this time," she began carefully. "That side of the story is...unpleasant to explain and relive-"
"Something your Mother did?" Rias asked carefully. She flinched back at the intense drop in temperature caused by the change in Satsuki's demeanor. "Ryuko insisted on a few conditions to becoming part of my Peerage regarding what I wasn't allowed to do, attributing her Mother as being 'the unusual' when she realized how...distasteful I found the idea of...using someone in my care like that."
Satsuki took a deep breath, then let it out, closing her eyes as she regained her composure. "You are right it involves our Mother, and that we will not be discussing her," she allowed calmly. "However, the simple fact is that...Ryuko is half-human, half Life Fiber." She paused as she saw Mako playing with Ryuko's wings. "Or...was."
"I have managed to deduce the change in Ryuko's Fibers," Shiro spoke up calmly. "They no longer utilize Life Fiber energy...or at least, not entirely. The energy they use has adapted to Ryuko's magical energy, and can utilize both it and Life Fiber energy. In addition, the Fibers of her uniform have also shifted, and...appear to be infusing the rest of her uniform with her energy, propagating by feeding on the ambient magical energy of the environment without harming it as well as her own excess. When it was made, it was 5% Life Fibers. Now, it is 6% of these...until we know more, I will mark them down in the files as D-Fibers. D for Devil. However, it is not entirely Devil energy as I am analyzing from Rias-san."
"She does have a Sacred Gear of some sort," Rias spoke up. "It can no longer be detected, likely meaning her Fibers used it to bridge between their nature and her new magic."
"Well, sounds like we've covered everything," Ryuko spoke up grumpily. "If you excuse me, I'm going to bed. Apparently, coming back to life is exhausting. Who knew?"
"You probably did already," Satsuki teased. "I'd say you've come close to doing so a time or two."
"Oh get bent, sis!" Ryuko snapped playfully as she and Mako left, heading for their room.
Rias groaned and rubbed her forehead. "Tell me honestly, Satsuki-san...how much am I likely to end up drinking over everything Ryuko's going to put me through?"
"Unless you mean tea, I'd recommend not starting," Satsuki responded calmly. "Assuming enough alcohol even exists, it would probably kill even a Devil." She suppressed an amused smile as Rias groaned and buried her face in her hands.