Late in the Arrancar arc of Bleach, Ichigo Kurosaki stormed the palace of Las Noches with his friends to rescue Orihime Inoue, since Ulquiorra Schiffer had abducted her and taken her there for his master. Ichigo faced and defeated Grimmjow, then faced Ulquiorra in a rematch and showed the true extent of his powers. Just when Ichigo seemed defeated, he came back as a Vasto Lorde, and turn the tables on Ulquiorra.
Not only did Ichigo bounce back from defeat, but he wielded power that slightly eclipsed Ulquiorra's fearsome Segunda Etapa, to Orihime's and Uryu Ishida's astonishment. Then Ichigo pinned down Ulquiorra and fired a heavy-duty cero blast right at him, but failed to finish him with that merciless blow. How did Ulquiorra survive that?
What Ulquiorra Didn't Understand
When Ulquiorra first appeared alongside Yammy in the world of the living, he was the epitome of a cold, ruthless villain. He coldly carried out Aizen's orders to help make Aizen's grand plan a reality. This set him apart from Yammy, Grimmjow and Nnoitora, who loved battle and mainly defined themselves by finding and defeating the greatest of enemies. Grimmjow, unlike Ulquiorra, longed to become a "king" and set out to destroy anyone who got in his way. That was his heart's greatest desire. But Ulquiorra, by his own admission, had no heart at all. He was a nihilist who denied the existence of anything he couldn't see.
This contrasted him sharply not just with Grimmjow, but also his prisoner Orihime Inoue. She anchored her hopes and courage on the heart that she shared with all of her friends, and it was her personal bulwark against Ulquiorra's and Aizen's intimidation techniques. This is similar to how Rukia bore the heart of the deceased Kaien Shiba, keeping a piece of him alive in her. Ulquiorra rejected this idea, but it fascinated him, and he longed to see what exactly Orihime had inside of her. Was the heart an organ? Could Ulquiorra hold it, see it, crush it? Whatever it was, it granted Orihime impossible power that defied the odds, sustaining her and Ichigo throughout the Arrancar arc.
As Ulquiorra battled Ichigo for the final time, he faced the power of Ichigo's heart, just like Grimmjow did. Even when Ichigo seemingly returned from the dead and became a Vasto Lorde, Orihime had faith in him, and Ichigo fell to Ulquiorra, Orihime was still crying out to him for help. There's the mysterious heart again.
Ulquiorra's Unfinished Business
Fast forward to the battle's end, and Ichigo had claimed the upper hand at last. His Vasto Lorde form was even stronger than Ulquiorra's fearsome Segunda Etapa, and Ichigo blasted Ulquiorra with a mega-powerful cero, right up close. Inexplicably, Ulquiorra survived this brutal finishing move, and likely for more reasons than the physical ones. On one hand, Ulquiorra had advanced regenerative powers, and even after taking a blow like that, he could consolidate and heal his flesh just enough to stay alive and functional, albeit barely. He managed to regrow most of his body (minus his vital organs) and dealt a retaliatory blow that dissolved Ichigo's Vasto Lorde form, then slowly crumbled away. Ichigo was distraught about winning "unfairly," but Ulquiorra had his own regrets.
It's possible that Ulquiorra's own desire to see the true nature of the "heart" sustained him, and he stubbornly refused to die before discovering the truth. He thought his way of life was supreme, but Orihime's was something alien and new, and it seemed even stronger. In his dying moments, Ulquiorra reached out to touch Orihime and see that enticing "heart" at last, only to crumble away as Orihime reached back at him. Not only that, but Ulquiorra might have begun to grow his own heart since he sought to see Orihime's and perhaps model his own newly-born heart after it. He also might have survived Ichigo's finishing move out of spite, like Grimmjow. At the last moment, Ulquiorra's young heart burned with feelings of rivalry, and it refused to go down so easily.
Ulquiorra didn't just survive for Aizen's sake -- he survived to protect his young heart from the humiliation of losing to a human who was only mimicking the supreme power of Hollows. In a way, Ulquiorra became like Ichigo himself, longing to connect to Orihime's heart and feel its warmth, while stubbornly refusing to go down in battle before a bitter rival. In that sense, Ulquiorra became his strongest self, just in time to crumble to ash.
