Ulquiorra is a slender, yet fairly muscular, male Arrancar of average height with a melancholic appearance, fairly short, messy black hair, pale white skin, a black upper lip, and green eyes with small slit-shaped pupils similar to a cat's. Part of his bangs falls between his eyes, and he has distinctively thick eyebrows. He has teal lines that descend from his eyes, making it appear as if he is crying. His facial expression rarely changes, and he is almost always frowning. He wears typical Arrancar clothes: a white jacket, black sash, and a white hakama. However, his jacket seems to have longer coattails than others, and the collar is rather high.
Like most other Arrancar, he possesses remnants of his former life as a Hollow in his appearance. His Hollow hole is located on his sternum, though initially, the hole was located on the base of his throat, a change that was seen in both the manga and anime. The remainder of his Hollow mask lies on top of his head, forming a broken horned helmet. He is the fourth-ranked Espada, signified by the tattoo on the left side of his chest.
Ulquiorra is a very cold, callous, dispassionate figure, and is rather aloof, brooding, and indifferent, willing to harm both his comrades and enemies should they ever get in his way. He refers to anyone he does not find interesting as "trash" and treats them as expendable. Despite this, like most of the other top Espada, he is not particularly violent and will only fight when provoked or ordered to by Aizen.
Ulquiorra demonstrates a strange habit when killing victims he is particularly interested in, stabbing them with his bare hand in the same location as his Hollow hole, as stated by Grimmjow. Whether he is consciously aware of this habit or not is unknown. During a speech to Orihime, he claims that he does not believe in Human emotions, which he refers to as the "heart," reasoning that if his eye cannot see them, then they do not exist. These words, and much of Ulquiorra's overall behavior, reflect the concepts of materialism, which also connects to his aspect of death as an Espada, Emptiness. In the last seconds of his life, he seems to finally find his own "heart" and understand feelings.
His cold demeanor allows him to stay completely calm and in control in most situations, and he is not easily surprised or caught off-guard. However, he does not seem to comprehend the Human trait of fighting against all odds or the very concept of the Human heart; during his final battle with Ichigo, he completely overwhelms the Shinigami after releasing his Zanpakutō, yet Ichigo continues to fight on regardless. Because of this he shouts at Ichigo out of frustration, telling him that "to continue fighting is pointless." He then transforms into his Segunda Etapa form in an attempt to show Ichigo "true despair." He also seems to contradict himself, telling Ichigo that although it is natural for humans to mimic Hollows to become stronger, they will never be the equal of Hollows, though the Arrancar are doing much of the same by mimicking Shinigami.
Ulquiorra is also highly perceptive, cunning, and analytical. During his and Yammy's first visit to the Human world, he makes several surprisingly accurate remarks on the Humans who confronted them. He theorizes that Orihime Inoue's healing power is a temporal-spatial ability, which Aizen later describes as the rejection of fate, and comments on how Ichigo Kurosaki's Bankai can easily cut through Yammy's defenses. He also notes that Ichigo's power fluctuates, varying between very weak and stronger than his own.
Ulquiorra seems to be demanding as well, such as when he came to Orihime's chamber and ordered her to eat a meal an Arrancar servant brought in, claiming that, for Aizen's benefit, it was her "duty" to stay alive. When Orihime hesitates, he threatens to force the food down her throat, or strap her to a table and feed her via an IV. This also implies that, while he prefers not to dirty his hands, Ulquiorra is fully capable of committing truly abhorrent and inhumane actions without feeling any shred of remorse or hesitation.
After Ichigo's Hollow form overwhelms and strikes him down during his last battle, Ulquiorra still maintains his composure and calmly berates himself for being defeated by a "Human-turned-Hollow." Believing that he no longer has a purpose in life since he was quickly defeated by his opponent, he demands that Ichigo kill him, and compliments Ichigo when he decides to use a point-blank Cero to kill him, claiming that the lack of mercy was "rather Hollow-like." This is a marked departure from most of his fellow Arrancar, who died cursing their enemies or with an abject fear of death.
Cero:
Ulquiorra has been shown shooting Cero from his fingers with excessively destructive power. His Cero can be charged and fired relatively fast and is a green color instead of red, which is the most common color of a Cero. The force of the blast is powerful enough to send an opponent hurtling long distances, effectively destroying any obstacle in its path at great range. He is shown able to fire it from either hand.
Cero Oscuras
(黒虚閃, (セロ・オスキュラス), sero osukyurasu; Spanish for "Dark Zero", Japanese for "Black Hollow Flash"): It is a black Cero with a green outline to it, which Ulquiorra states to be similar to Ichigo's black Getsuga Tenshō. This Cero presumably can only be used while he is in his release state. Ulquiorra's Cero Oscuras is powerful enough to blast Ichigo's mask away and destroy a large part of the city-sized Las Noches' dome. Ulquiorra refers to it as "our Cero". It can also be used in the Segunda Etapa form.
Resurrección: Segunda Etapa
(刀剣解放第二階層 (レスレクシオン・セグンダ・エターパ), resurekushion segunda etāpa; Spanish for "Resurrection Second Stage", Japanese for "Sword's Release: Second Level") Upon his transformation, Ulquiorra boasts that he is the only Arrancar among the Espada who is able to reach a second Resurrección form, and that even Aizen has not seen him in this form. Ulquiorra refers to his second state as "True Despair", both before and after its release, because the immensely dense spiritual pressure it releases instills despair upon those around him. While he retains his black wings, his long white coat is gone, revealing a more defined musculature with his chest bare and Hollow hole (which has become larger) dripping black, blood-like liquid. His waist becomes covered in something similar to black fur, which also covers his arms and legs. His fingers grow claw-like extensions and his feet become like talons. The irises of his eyes turn yellow while the sclera becomes green. The tear-like marks under his eyes increase in size and turn black. Ulquiorra's mask is now completely gone, as he has taken on the fuller appearance of a bat. He now has two large bat ears, and the #4 Espada tattoo is no longer visible on his chest.
Info from : http://bleach.wikia.com/
