Bleach's arch-villain is the ex-Captain Sosuke Aizen, and his ambitious scheme called for an evil army of Arrancars or maskless Hollows. Not just any Hollow would do; Aizen needed the best of the best, and the Espadas were made up of Hueco Mundo's scariest Hollows of all, such as Ulquiorra Schiffer.
Ulquiorra was one of the first Arrancars to appear in the story and at first, he seemed like a simple inversion of the Soul Reapers, having a white uniform with a black belt and a zanpakuto (and the remains of a Hollow mask). But bit by bit, Ulquiorra revealed just how unusual he was, and he was capable of things no other Arrancar could imagine.
Ulquiorra's Regeneration Is Almost Unparalleled
One of Ulquiorra's greatest assets is his ability to regenerate his wounded flesh, especially if he is in his released modes. Any Arrancar can slowly heal over time, as can the Soul Reapers, but not on-demand. Yammy Llargo got his arm cut off with Ichigo's bankai and he needed a medical Arrancar to sew it back on. Ulquiorra, by contrast, can rapidly restore any flesh that is injured or even clean torn off, giving him remarkable battlefield endurance in addition to his hierro skin (one of an Arrancar's many natural abilities). For instance, Ichigo once tore off Ulquiorra's entire arm and it grew back in seconds. Even when Ichigo very nearly blasted Ulquiorra to smithereens, the 4th Espada regenerated his head, torso, and arm and a leg to keep fighting (though only for a short time).
Ulquiorra Has a Magic Eye No Other Arrancar Can Match
Ulquiorra is a scout as well as a fighter, often sent to spy on Aizen's enemies and report back with vital intel. This was his first on-screen mission, in fact: to assess Ichigo's current power level. When Ulquiorra returned to Las Noches (Aizen's palace) to make his report, he used more than words to explain what he had experienced. He removed his eyeball and crushed it, and the floating magical power infused itself into the minds of everyone present.
Aizen was able to see, hear and feel everything that Ulquiorra had -- a sensory power that no other Arrancar can match. It's true that Aaroniero Arurruerie has an unusual sensory sharing ability too, but it wasn't quite as fancy as this. And since Ulquiorra can regenerate flesh rapidly, he was able to restore his crushed eyeball before long.
Ulquiorra's Hollow Hole Dictates Where He Lands Hits
All Hollows and Arrancars have a round hole somewhere on their bodies, most often somewhere in their chest. Grimmjow's is roughly where his belly button would be while Ulquiorra has his right around the hollow of his neck. In his first appearance, Ulquiorra even unzipped his uniform just enough to reveal it, for dramatic effect.
What's strange, though, is Ulquiorra's habit of striking his enemies bare-handed in the same spot on their body where his own Hollow hole is found, as though making a unique imprint on his fallen foe. Perhaps it's a gesture to mean "now you're as empty as I am," or it's a habit that Ulquiorra isn't even aware of. Grimmjow noted this habit but was unsure whether Ulquiorra did this on purpose. It might just be reflex.
The Reason For Ulquiorra's Black And White Body
Hollows always have white masks to represent the stark white bones of the dead. As for their bodies, there's some variance, such as the dark green fur of Grand Fisher or the black robes of Barragan Luisenbarn, a noted Vasto Lorde Menos. Ulquiorra, as a Vasto Lorde, had a white body, and that made him an outcast among his fellows, who had black bodies. Ulquiorra was rejected by them and wandered Hueco Mundo until he managed to find peace. This gave him two release forms: the first has a white body while the second is a black one, like his old friends had.
Ulquiorra Has Two Releases
Speaking of which, it is highly unusual for Ulquiorra to have two released forms, and what's more, Sosuke Aizen is completely unaware of this fact. It is possible that Ulquiorra would have explained this to his master someday when he felt ready, but that time never came. Ulquiorra's zanpakuto is Murcielago, or "bat," and it grants him a white-robed body with bat wings, a two-horned helmet and new abilities. This form alone was enough to dominate Ichigo in battle, but then he performed his Segunda Etapa, or second stage, and showed off his final form.
He had a slender, black-furred body with a long tail, impish feet, and no helmet -- just his horns. He retained his wings from his other form, and his Hollow hole became even larger. In this form, his eyes are green and yellow, and the tear trail marks on his face became even thicker than ever. Ulquiorra boasted that this state could induce absolute despair in his enemies, and in this stage, Ulquiorra's regenerative powers are at their height.