Reiryoku: the amount of spirit energy an individual posesses. Acts like fuel for all those otherwordly ability. It's the equivalent of the real-world "potential energy".
Reiatsu: the pressure an individual emits when making use of his/her reiryoku. Every being has it, including humans. It is not the power per se, but a "signal" every spirit being emits. This signal is usually unconscious, but can be suppresed for stealth purposes or augmented in order to crush a weaker foe. If the reiryoku is the stuff from which power comes, reiatsu is the way people in Bleach measure it. For instance, somebody would say regarding Ichigo: "I can feel a huge reiatsu coming from this guy. His reiryoku must be off the charts". If Reiryoku is energy, reiatsu is "power" (that is, energy being put to use).
Reishi: the stuff spirits are made of. If atoms are what make matter, reishi is what makes souls and objects in SS and HM. Zanpakutos, Kidos, Hollows, and Fullbrings (but NOT Fullbringers and Quincies while in the human world), as well as buildings and trees in SS and HM are made of this.
Quincies absorb reishi and combine it with their own reiryoku to fight, a process that generates a reiatsu signal just like everybody else does in this manga. Shinigami (which are made of reishi) combine their own reiryoku with reishi they create to fight. Both of them have a reiatsu signal; the difference is that Shinigami and Hollows tend to release huge amounts of reishi when fighting (that's the reason for all those fancy explosions and smokescreens after a bankai or Resurrección), while Quincies don't.
So yeah, Yamamoto should have been able to read Yhwach's reiatsu. I guess Royd could mimic reiatsu (or at least he could mimic the signal), but not reiryoku.
I would argue the reishi wave orihime referred to is basically reiatsu. I mean, what would be the difference? Doesn't reiatsu (spirit pressure) refer to the volume or concentration of reishi/reiryoku emitted by someone making reiatsu basically an application of reishi/reiryoku? There is also the consideration that the final form used by ishida is different from the holy form used by kirge. I would argue that the simple answer is that the lets stilt does emit reiatsu while the volstandig does not.
Ishida became immensely powerful at that moment, but he was still less experienced. I would think that the most skilled Quincy are so adept at manipulating the surrounding reishi that none of it goes to waste. So when Quilge went Vollstandig, every bit of reiatsu he was emitting was funneled directly where he wanted it to go. In Ishida's case, he was using that power for the first time, plus he was using ransotengai (the puppet string thing), so he was actively surrounding himself with reiatsu to direct his body. On top of that, I'm sure he wanted to intimidate Mayuri, so he was letting his reiatsu flow freely.
I just got another idea about mysterious person in HM and Squad 0. It could be Hikifune. She must be strong as she got promoted to Squad 0 and Urahara had no idea of her strength. She must be at least stronger than Ukitake/Shunsui/Unohana, because it her who got promoted, not them.
http://www.mangareader.net/bleach/491 She said reiatsu... Unless it is mistranslated, an easy way to get around this conflict is that assuming Letz stil is different than Vollstandig in term of reiatsu emission.
As far as hikifune being stronger or weaker than anyone in particular... I don't think we have evidence to even have a proper opinion on the matter. She seems to have been a surviving member from the old guard from 200 years ago however that does not necessarily imply she is at any given level. So far the speculation on the royal guard is that they consist of captain level shinigami however we don't actually know how many members they have or just what the qualifications required are. We also don't know if any of the captains from the time either rejected the position or if they were considering skills other than sheer strength or what position exactly it was that hikifune got in the royal guard. Kyoraku has several personality flaws, ukitake has a disease which strikes precisely when the plot demands it and unohana would be hard to replace in the squads.
So it could possibly be a member of RG. But then it could also be an Arrancar who managed to avoid the Vandenreich's hunting squads.
Isshin was Squad 0 as far as we know, and he is a monster even after being rusty and not fighting for a looong time. And I don't know if Yamamoto even knew about it. It's like he was just there without promotion. But Gin knew about Ichigo. Maybe from Aizen. When he fought Ichigo for the first time he said "Kurosaki Ichigo? So he really does exist..."
Why would Squad 0 go there? Because they know SS can't win, because they were pretty much screwed in HM and Isshin is missing, so he might be persuading them to help.
There are still plenty of possibilities regarding ishin though. He could still be a member of the royal family, a member of the shiba clan, an ordinary rogue shinigami, the late 10th squad captain mentioned in the TBPA...
Isshin is well known among the (oldest) members of the Gotei 13, such as Kyoraku, Ukitake, Unohana and Old-Man himself. Kubo said, that the reason why Urahara brought the sword to Seiretei to get back Ichigo's powers is because if Isshin went, some things would have been revealed for the Gotei 13. Isshin was possibly a Squad 0 or he got in conflict with the Gotei 13 at some point (maybe 1000 years ago?) and he had to leave. I wouldn't be surprised if it turned out that he had to save someone, maybe a Quincy (Masaki? Her father - Juha?) from Yama and thus he had to leave. Don't forget that one point it is highly possible that he used the FGT... I doubt that it was 1000 years ago, he doesn't seem that kind of guy who lived in the world of living for such a long time, but who knows. Maybe he did.
Maybe Yama-jii referred to him, when he said, no stronger shinigami was born in the last 1000 years. I mean Isshin flicked Aizen through some buildings!
Hikifune is quite a possible choice for someone to appear in HM. Grimmjow couldn't do much this time, but Hikifune... She may have information, strength, everything that is needed for some action. (On the other hand, Grimmjow has fanservice in his pockets, and Neliel turning into an adult is also a possible outcome. She has boobs.)
It is really possible for Isshin to be a member of the royal family,thus it would explain why Ichigo's and Isshin's Zanpakutou are almost the same and why Ichigo is so special[bloodline],I doubt he's a member of the shiba clan[Ganju didn't recognize the name Kurosaki],I doubt again he's an ordinary rogue shinigami since when he confronted that Arrancar to save Kon,he explained the basic of the size of a Zanpakutou,displaying knowledge about Zanpaktous and captains.Quote:
Originally Posted by kkck
Most likely outcome is that Isshin is some kind of captain [either in Gotei 13 either in the Royal Guard either the one mentioned in TBPA],because he has that weird captain haori hanging on his shoulder;or rather piece of a haori.
How come ? I must have missed that part,'cause I don't recall anyone besides Ryukken and Urahara knowing Isshin.If the senior captains and Yama-jii knew about Kurosaki Isshin,how come they didn't recognize Ichigo when they heard about him for the first time ? [Kurosaki Ichigo].I don't know why,but I doubt this is the case.Quote:
Originally Posted by Kuchikido
Regarding the one holding the Zanpakutou to Urahara's face,there are plenty of possibilities : Isshin / Hikifune / Grimmjow / Nel / [random RG] / [random Arrancar] / Gin [though he is most likely dead,but then again,Hyiori cut in half and surviving is ...] .Tbh, I doubt it will be a RG.I mean,wtf,a fight between Ichigo post-Dangai and evolved Aizen [his last form was so...Hollowish,exactly what he despised],with the world at stake didn't take them out[and then Yama-jii had fallen too,along with all the captains and vizards],so ...it's really unlikely to see a RG[imo].