He is a man of literal legend. He's lived since before the begining of time, as we know it, and has stayed alive for so long only because of the Fountain of Youth.
Zero is a hero by technicality, but a villain by law. He's mostly calm and couldn't give a rat's ass about anything, but will listen to stories and how things came to be, especially during a fight or war. He is a violent maniac that can, and WILL, kill anyone that interferes with his battle or gets in the way of his attacks. He only cares for his sister, Yuki (who was previously thought to be named Ayumi), and his brother, only when he was nearing the end of his life and Zero knew the truth about him. He cares somewhat for others like his best friend, Metal Claws, his allies, Yukujin, Suijin, Wolves Bane, Sin, and Jesus, but mostly doesn't care for anyone else, even the people who train him. He is thought to be cold-hearted, souless, and a muerderer, but Zero will always put justice before his own pleasure. When he is using the power of Sensless Sinless, depending on who he takes Sin of, he is usually more cocky about his power and will try to put others down. He might also be happy most the time and self centered about his own strength that he might block out what others do or say.
Zero carries around a sword that has been sealed with the soul of a human who is known as Paul. The sword speaks to Zero when he is in grave danger or has reached a level of achievment and gives him more power and abilities. A list of the abilities that Zero gains is listed below (NOTE: These are abilities he gets from his sword):