Ulquiorra Cifer, the fourth Espada in Sosuke Aizen's army. He was not always known by this name. The fourth Espada was known for his despair, which perhaps was a result of him ultimately committing suicide in his past life over his lost love. Perhaps there were other factors that drove him to despair. Perhaps he had a strict family and was told to uphold tradition. There are many possibilities. Let us examine one of them…
The fourth Espada always gave off an air of grace. The way he spoke and behaved hinted that he used to be someone of important status in his past life. It's possible that he wasn't human, but a soul with powerful spiritual pressure who had already been residing somewhere in the Soul Society. Ulquiorra was a kind-hearted soul. He knew that others looked upon shinigami and hollows as different as day and night. Despite this, he wanted to form an alliance between the Soul Society and Huecdo Mundo. He was even against the extermination of the Quincys. Ulquiorra's views were so different from the tradition that he was raised to uphold, it resulted in conflicts between him and his family.
While trying to get his family to see things from his point of view, Ulquiorra met a village girl who became the love of his life. This village girl was the former life of Inoue Orihime. After several meetings, Ulquiorra revealed his true identity to her. She was shocked at first, but did not criticize herself for daring to love him. The two made plans to be together forever. However, Ulquiorra's parents found out he had been seeing a village girl and were furious. His father ordered that he stop seeing this girl, but Ulquiorra defied him. One night he snuck out of his home to see her but was met with resistance from the guards. After a bit of fighting he was able to slip past them but was apprehended by his father who tied him to the spot with kidou. Ulquiorra was unable to break away. He resented his father.
Ulquiorra became a prisoner in his home. There were guards surrounding his room. He refused to eat and drink (resulting in his arrancar form being pale and skinny). His parents decided to arrange a marriage for him in order to make him forget about his love. While everyone was busy with the wedding preparations Ulquiorra was able to escape. He had already decided to sacrifice everything that he was and will be all for the sake of love.
He came to the village of his love only to learn that she passed away and had already been buried. Grief overcame him as he found his way to her tombstone. Tears flowed as he cried out for her (the tears in his arrancar form). There was nothing left for him. He had abandoned what he was born to be and what he would've become, and he had lost the one person he loved. He unsheathed his zanpaktou and stabbed himself in the chest (the hollow hole).
Due to Ulquiorra's mass spiritual pressure he was able to maintain most if not all of the memories of his past life, but he had lost his heart. Ulquiorra was able to become an arrancar without the help of Sosuke Aizen. Despite being a hollow now, Ulquiorra did not give up his dream of uniting Huecdo Mundo and Soul Society. He traveled throughout Huecdo Mundo and gathered supporters. During the time he was gathering supporters, he heard that Aizen was gathering arrancars on his side to start a war. Ulquiorra did not like Aizen's manipulative and bloodthirsty ways. He feigned loyalty to Aizen and joined his army in order to infiltrate his forces. Ulquiorra knew of Aizen's ability to create illusions. Ulquiorra's right eye would see what others under hypnosis saw. His left eye, after being crushed and regenerated, would be immune to Aizen's ability. Ulquiorra's plan to spy on Aizen was put on hold when Ichigo's new hollow form caused him to activate his regenerative abilities. As he turned to ash, memories of his past love flooded his mind. Looking at Orihime and reaching his hand out to her, he remembered who she was. Ulquiorra went through a "rebirthing" stage. He left Las Noches and went back to his supporters to inform them of his findings.
On the dome of Las Noches, Uryuu was healed by Orihime and went to join Byakuya and Kenpachi in the fight against Yammy. Orihime attempted to use her powers to resurrect Ulquiorra, but nothing happened. She wept, believing that she couldn't do anything. It was through no fault of hers. What was never dead to begin with cannot be brought back to life…