The Zoldyck Family first rose to power as independent Hunters. They developed tools, weaponry, and skills that can allow humans to fight monsters from the Outlands on something resembling equal footing. After the first head died, though, the family changed directions - they hunted monsters less by duty and more by contract. When their need for money and power increased, they expanded their services to apply the same skills against humans when the price was right. Today, they stand as one of the world's most powerful assassin organizations, known by many who have ties to or interest in the criminal underground. Despite their relative lack of secrecy, most law enforcement leaves them alone for fear of lethal retaliation from the family.

Like everyone who grows up on the estate, Killua was trained in assassination arts from an extremely young age. In addition to training that threatened his life constantly, he was forced to undergo all kinds of torture at the hands of his brothers in order to build up his body and mind's resistances to them. Despite the brutal upbringing, Killua thrived, carrying within him a talent for killing that rivaled any of the greatest assassins in the Zoldyck family tree. There was only one problem - he didn't actually want to become a professional assassin when he became an adult. At age 12, this schism led to a fight in which he injured his older brother and mother before running away from home. The family wasn't especially troubled by this - on the contrary, his mother was delighted that he had such talent that he was able to harm her at such a young age. The family is, for now, letting him get the wanderlust out of his system, with the quiet but forceful assumption that he'll eventually return to them and join the family business.

Shortly after fleeing his family home, Killua was approached by agents of Daybreak Academy who foresaw the potential within him. He accepted their offer, and enrolled as a student.

• Extreme Perception: Killua's senses are trained to the absolute limits of what humans can achieve without supernatural help. He can see and hear things normal people would miss, he can (sometimes) sense the presence and killing intent of hidden people around him, and his reaction speeds border on superhuman.
• Extreme Agility: Thanks to his assassin training, Killua can move at incredible speed, both over long distances and with small precise movements, and his strength and endurance are both above average, especially for his age.
• Combat Proficiency: His specialty is hand-to-hand combat, but he's also highly skilled with a specialized signature weapon - a pair of yo-yos made of a heavy and highly durable metallic alloy. At the moment, at least, he's more confident with his unarmed combat skills.
• Pain Resistance: Killua underwent brutal torture-resistance training as a child, and can now take all but the most agonizing of wounds without blinking. He's built up immunities to many of the most commonly-used poisons as well.

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