Alright, wrote most of this stuff in the chatbox but going to also post it here. These are just some assumptions and decisions I have made about reishi in general.
First, the big one.
Reishi affects the material world, and the material world can affect reishi. What does this mean?
Well, it means no phasing through people or buildings. It means that if you are punched through a building you will break that building. It also means that if a nuke is dropped it will at least hurt any spiritual entity and kill any but the most powerful.
Second thing, reishi based entities or abilities are much stronger than those on the physical plane. So it is logical that Levi could shoot a bullet at a shinigami. However it would do nearly nothing unless it was a really weak shinigami.
Why is this? Well I have my own theory. Atoms outside of when they have some type of charge bond, do not like being near each other. While the nucleus is really small, there is a much larger field of negative energy around each atom due to the electrons that move around it. These repel other atoms which keeps your hand from going through a wall or something because the negative field on the atomic level keeps the two from crossing, even if the solid nucleus does not.
So reishi based beings either have a lesser level version of that field, or more likely are just solid particles. So they still can't pass through each other but are able to bond stronger to each other. They also can be compressed to extremely high levels.
Third thing is my personal idea behind spiritual pressure. I kind of see it like what happens with regular particles on earth. The more particles, the more mass. The more particles in a given space, the higher the density. And as density increases, the particular gravity field around an object also increases. So with a spiritual body, the more reishi they have under their control the greater their spiritual pressure. And when they concentrate their reishi and their power, the pressure can increase even more.
(That statement probably contradicts other things in Bleach but I really don't care that much. Was just a random idea that popped into my head i decided to go with.)
Last thing I noticed-
Shinigami, zans, kido - pure reishi
Hollows, arrancar, ceros - pure reishi
Quincy bows, the outside of their swords, and the inside of the ginto - pure reishi
Superhumans...
they tend to break the rules a lot. Some of them like Akutore still follow the basic logic of making strings out of reishi and using those reishi based weapons.
Others however have managed to do something never seen before. Levi shall be the example. He litterally changes his body to make two large bone cannons come out of his shoulders.
So this happens in one of two ways. He uses reishi to physically manipulate the atomic structure of his body, something that no one has ever done before. Or, he instead changes parts of his shoulder from atom based to reishi based, and then manipulates that reishi into the new form of the cannons, and then changes it back to an atomic based structure.
(BTW, I prefer the latter explanation because it makes a little bore sense. Reishi has been shown to be much easier to manipulate properly than atomic based things. It molds itself to the way the user imagines it better and will probably be the decision I actually make.)
Either way, this is something that I have never really thought about before. Before I have just been handwaving and saying "whatever you want as long as it is not OP". However, there may actually be a rhyme and reason to what the superhumans can do. Considering that either way I decide with this, it might lead to interesting plot points as the shinigami or some of the other races try to see if they can learn how to do that.
So discuss if you want to or just wonder why I waste so much of my time on stuff like this.
Wed Jan 02, 2013 5:26 am by 
