Who is Ulquiorra in "Bleach"?

Ulquiorra Cifer is a male Arrancar from "Bleach." He is a rank 4 Espada in Sōsuke Aizen's army, which consists of the most powerful Hollows under Aizen’s command.

Human souls that refuse to cross over and remain in the human world turn into Hollows. Arrancars are Hollows that managed to remove their masks and obtain powers similar to those of soul reapers. Ulquiorra was an albino Hollow and was shunned by other black Hollows. He hated himself for being different and was alone until he found a white tree with sharp spikes. He was drawn to it and destroyed his mask by sinking himself into the sharp spikes. Some time later, he was recruited by Aizen to fight by his side.