Why is Ulquiorra the only Espada with a second release? 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:


Why would he not show Aizen how powerful he is?
Why is he the only Espada with a second release?
If Aizen knew about his second release, where would he be in the Espada rankings?
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