In Bleach Ulquiorra is the only Espada with a second release, and he admits he has not shown Aizen. So I have come to a few questions:
Answer to first question:
A. Just in case.
Ulquiorra’s a pretty careful guy. He likes to make plans. And stick to them. So it’s possible that he hides his second release just because he knows that it’s good to have an ace in the hole, just in case he ever needs it. Not that he’s planning to use it for any nefarious purposes - he just figures, why tell if he didn’t have to?
Ulquiorra: Also, I’m not really a talker.
B. As a tiny act of rebellion.
Or you could read Ulquiorra as more of a Grimmjow-figure. You know, someone may recognize that Aizen is too powerful to actively rebel against, but who needs to do SOMETHING a little rebellious just to stay sane. Grimmjow leaves meetings early and sneaks off to the world of the living; Ulquiorra hides his second release. And this allows them to put up with working for a freakin’ shinigami.
Grimmjow: Um…I’m pretty sure that rebellion doesn’t count if nobody knows about it.
Ulquiorra: And that’s why you get spiritual pressure slammed to the floor, and I don’t.
C. Because he was actively disloyal to Aizen.
It’s possible, though, that this wasn’t a tiny act of rebellion, but rather a big one - that Ulquiorra had some sort of secret plan to rebel against Aizen someday, and he was saving his second release for that time. In which case letting Aizen go off to the WOL with the top three espada was a damn good plan. Ulquiorra would have had the castle to himself if he hadn’t been killed by Ichigo. So close, Ulquiorra.
Ulquiorra: And I was really looking forward to sitting on that throne, too.
Aizen: Yeah, white marble is comfier than you’d think.
Source: "Why does Ulquiorra keep his second release secret?" on bleachlists.tumblr.com
Answer to second question:
I came across an answer while scavenging the net (and I think it's pretty good):
Well..the arrancars sword and the soul reapers swords are the same except for how they are released or how to manifest themselves after they are released. Arrancars have the power of soul reapers and soul reapers swords have 2 different forms shikai (1st form) and then bankai (2nd Form or release state). Now most Arrancars only have 1 sword release thats their resurreccion, resurreccion means resurrection and if u notice all arrancar release states are in spanish for some weird reason, but they only have one b/c arrancars have only been around for only about 1 year or a couple of months b/c they didn't come into existence till azien stole hogyoku out of Rukia's chest. Seeing how they're a new form of hollow but with soul reaper powers they probably have not explored all of their powers unlike soul reapers who have been around for over a 1000 years which means they would know more about their swords and how they operate. So basically the 2nd form Resurreccion segunda, which means 2nd resurrection, is the 2nd form of the sword similar to a soul reapers bankai. So in other words Arrancars haven't been around for a while which is why a lot of them don't know the other form unlike soul reapers who know both forms b/c they've been around forever. Now every arrancar has one but Ulquiorra was just the first to learn it.
Source: "Why does Ulquiorra have two forms?" on Yahoo Answers
Answer for Third one:
Ofcourse Aizen knew.
...[m]aybe Ulquiorra just wanted to know how much Aizen really watched, or how much he could just deduce by being insanely smart. So he kept his release hidden to see if Aizen could figure it out for himself. And like any guy who tests a god, this just didn’t end well for poor Ulquiorra.
Aizen: Is there really any doubt that I knew?
and about the ranking:
Maybe he wanted to stay out of the top 3.
Maybe Ulquiorra just didn’t want to be in the top 3, and knew that with his second release taken into account, he would be. Ulquiorra’s smart - he might have guessed that the people who went with Aizen to Fake Karakura Town wouldn’t be coming back. Or maybe he just really likes the number 4.
Ulquiorra: Actually, I just know how important the number 3 is to Halibel.
Halibel: …
Ulquiorra: Don’t worry, we’re not having a moment.
Halibel: Oh, good!
Source: "Why does Ulquiorra keep his second release secret?" on bleachlists.tumblr.com
He had no motivation to show Aizen his true power, he was never shown to have any aspirations to become stronger or be recognised as the most powerful like some other characters, he always spoke about his and other's strengths as if they were just facts.
He was annoyed because he was so clearly stronger than Ichigo that it seemed stupid to him that Ichigo wouldn't give up, that's why he showed him his second release. He was already absolutely trouncing him in his initial release, it wasn't like Goku going Super Saiyan to defeat Frieza out of necessity, it would have been like Goku then going Super Saiyan 2 and ripping Frieza's limbs off one at a time.
He's the only one with a second release because he's the only one who figured out how to go further, again becoming stronger in your first release is probably completely separate to working out how to go to second release. There's no reason to think it's unique to him, although it is possible, it's more likely that he was just the first to figure it out.
I'd love to say that if Aizen knew about his second release he would have been the top Espada, as to me he seemed the most powerful and the coolest. Considering the ridiculous extent to which he was smashing Ichigo it's hard to believe he wouldn't have been at least above Harribel, who never really did anything overly impressive.
Personally, I think Ulquiorra in his second release was the most powerful thing we've seen so far other than Aizen and Ichigo post-Dangai, but I'm sure many people and probably even the author would disagree with me. The rankings were always a little dubious anyway, again, Harribel was pathetic, and it was absolutely bizarre when Yammy claimed to be the top Espada, and then was killed by Kenpachi and Byakuya, offscreen, without any apparent effort.
Saying he never showed Aizen his second release, does not mean that Aizen did not only know about it, but most likely talked to Ulquiorra about it at some point, we have seen that while at times Aizen can get very arrogant at times, like in his final fight with Ichigo, he is a genius.
He would have figured out that while soul reapers have their skikai and bankai, arrancars would most likely be able to have their own version of bankai, just as their first release is similar to a soul reapers shikai. The way Ulquiorra spoke when he said he never showed Aizen his 2nd release was vague, as he didn't exactly have to show Aizen his power, but he might have told him and made a plan with Aizen to keep it hidden from Gin, Tosen, and the other espada in case they rebelled, much like how Nel regained her power as an espada and starting to fight, and how Grimmjow went against orders to fight Ichigo, except to a more extreme level like a full scale rebellion. Aizen had rarely fought, at first having his espada fight for him.
If most of the espada went into full scale rebellion and if he knew about Ulquiorra's second release before hand, he would be able to send Ulquiorra out to deal with the treasonous espada and end them. It would not be that far fetched, considering the fact that Aizen seemed to hold a special trust in Ulquiorra that he didn't have for the other espada, sending Ulquiorra with Yammy to the world of the living, sending Ulquiorra to get Orihime after letting him trick the treasonous arrancars to steal a fake hogyoku only to get killed in the world of the living, entrusting care of Orihime to Ulquiorra, and lastly ensuring the safety of Los Noches to Ulquiorra.
It's possible that he would have also used Ulquiorra to kill Tosen or Gin if they rebelled, sense Ulquiorra's second resurrection was powerful enough to completely stomp Ichigo when he was using bankai and hollowfication. And as several others have said, it is most likely that Ulquiorra was just the first espada to discover and use the second release, Aizen not revealing to other arrancars, or even just the other espada, that it is possible for them to do this as they all seemed to be more loyal to their fraccion or to more loyal to themselves, unlike Ulquiorra who seemed to not only have no faccion, but never wavered in his loyalty to lord Aizen, you could say that in comparison that Ulquiorra is like the perfect soldier.
Not to mention IF Ulquiorra did aspire to rebel against Aizen and take his place, he most likely could have done so before Aizen was able to finally use the hogyoku. Not only is Ulquiorra already an expert in sonido and extremely fast without any form of release, in his second release he got faster, more agile, stronger. If he truly wished to rebel against Aizen, sense as far as we know Aizen actually never showed the espada his zanpakto, he could have completely blind-sided Aizen when ever he pleased, releasing his second release and using a grand rey cero while in his second release.
If he did this quick enough he would have been able to completely destroy Aizen, as we saw how powerful Grimmjows ceros were when he was just using the standard at point blank range and without his first release, Grimmjow not being as strong as Ulquiorra and with Ulquiorras power being enhanced with the second release, a grand rey cero at point blank range in his second release and if Ulquiorra put everything he has into it, if Ulquiorra was able to do this and blindside Aizen, well Aizen would have had no chance to survive it. And the fact is, Ulquiorra is very intelligent and would most likely realize that if he wanted, he could kill Aizen if he did blind-side him. But he didn't and was trusted with many responsibilities that the other espada weren't.
In conclusion, yes, Aizen more then likely knew about it and talked to Ulquiorra about it, deciding to keep it between themselves, yet would keep Ulquiorra at number 4 sense it would seem strange if the 5th best espada, I say fifth because while Ulquiorra was numbered at 4 the numbering system for espada was 0-9. Even the image above "Aizen-sama has not seen me in this form", the wording meaning that while Aizen has literally not been shown the second release Ulquiorra most likely talked to him about it, its possible that Aizen is even the reason that Ulquiorra got his second release before the other espada and another reason why Ulquiorra could be so loyal to him. Sorry for the very long answer.
although he is very smart, fast, and powerful, I don't think that he could win the number 2 spot from Barragan. to be honest, I don't even know how Barragan didn't get the number 0 spot from Yammy, considering that he could neutralize all of his attacks. although I am praising Barragan, I really think that Ulquiorra should have been sent to the world of the living, because of the fact that he could use his lightning lance and completely obliterate all four of the pillars within seconds. There is actually a Japanese comic written about Ulquiorra's backstory, and in it, he is actually a natural-born Arrancar just like Stark. With this in consideration, he must have been one of the original Espadas so he would have time to search his powers. The reason he said Aizon doesn't care, in my opinion, is because Uliquiorra's form of death he governs is nihilism. this means not caring about life, so I think that he just doesn't care if Aizon cares so he just doesn't tell him or show him. The reason that he is so loyal to Aizon is that he doesn't care about people or subjects so he is completely fine with stepping out of the way and letting someone take over as long as they are smart and powerful. if Soske was weak then Ulquiorra wouldn't let him take over. Back to his backstory, Soske found Ulquiorra in a big quarts tree in Hueco Mundo (like all other trees but way way bigger) they don't exactly say that Aizon found him there but after Ulquiorra claws his way into the middle of the tree which was tearing off his mask and making him an Arrancar (after recognizing that it was like him in that it was grown from nothingness) he found peace. I assume that Aizon built his palace around the tree making it like a masterpiece or felt his massive spiritual pressure inside of the tree, so he destroyed it. back to the point, when he was sitting inside of the center of that tree he probably started meditating ( like Ichigo in the Dangai ) and learned how he came to be and about himself and his power after this Aizon probably found him and took him out of the tree somehow. if he was in the tree for say 1 year, he would be older then any of the Arrancars, assuming that he was only there for 1 year, when in reality he could be their from anywhere from 50 to 100 years and maybe even more
If aizen new about gin's betrayal then I am pretty sure he new about ulqiuorra second release state and the reason why he has that is because he is an arrancar and arrancar are soul reaper/hollow hybrids like the visoreds but in this case arrancar are 60% hollow and 40% soulreaper. And since this is the case ressurccion is similar to a bankai so basically is first release is like shikai and the second works like a bankai. Not only that the espada are closet to a soul reaper than any other arrancar also the 7th espada his ressurccion stated by byakuya is similar to kido
My personal opinion on why he never showed Aizen his true form is because he was being calculated. Remember, he was easily the most calculated Espada member, in my opinion, easily able to match wits with anyone. Thinking that way, he could also match wits with Aizen, given his powers, intelligence, patience and lack of feelings towards others. He and Aizen were a lot alike. He may have had the same thinking that Shinji had: keep your friends close and keep your enemies closer. I believe he kept it as a trump card just in case he and Aizen faced off.
Ulquiorra is indeed the strongest espada. We all knew how Aizen had everything planned from the start- with the help of hogyoku, he'd transcend into a higher being and when he does, he'd want to test it against someone who had already transcended into a higher being. That being is ichigo kurosaki. But as aizen is already about to trascend into higher being, he had to give ichigo the time to be able to unleash the higher being within him which turns out to be the vasto lorde form then ultimately the final getsuga form. This is the reason why aizen had ulquiorra guard las noches so he could ultimately battle ichigo. Aizen might not have seen ulquiorra's segunda etapa but he must have known that he is far more powerful than number 4 suggests. The proof to this is when Aizen told ichigo "the fact you are here means you have defeated ulquiorra". Now deriving from aizen's judgement that is guided by the hogyoku, i can say that ulquiorra is the strongest among the espada and so strong that he was the one chosen by Aizen to bring out the higher being within ichigo.
1: Ulquiorra is intelligent. I believe he knew that in the final battle the stronger soul reapers would fight the stronger Espada Arrancars. That is why he didn't want to be higher ranked because then he'd have a lower chance of surviving.
2: Like I said before, Ulquiorra is smart so he is more likely to catch on to a second release/Bankai than others who were lazy or not as smart as Ulquiorra. I also believe Ulquiorra's cold and calculating personality had a part in it, as soul reapers' Zanpakutos are meant to be similar in personalities. If this applies to the Arrancars then his Zanpakuto will be more willing to show him his Bankai.
3: He would be number one, because for soul reapers the difference between Shikai and Bankai is extreme, this will definitely apply to Arrancars as well.
