Killua Zoldyck, born into the infamous assassin family, was raised to inherit its leadership. Forged through brutal conditioning granting immunity to poisons and electricity alongside intensive combat training achieving superhuman physicality, his upbringing demanded obedience and detachment. His brother Illumi enforced this through psychological controls suppressing emotions. Yet Killua harbored a deep aversion to killing and a longing for normalcy, leading him to flee home and seek the Hunter Exam as both challenge and escape.

There, he formed a pivotal friendship with Gon Freecss. Gon's unwavering trust and lighthearted nature offered Killua his first genuine companionship, central to his development as he prioritized Gon's safety above all. Initially failing the Exam after killing another applicant under Illumi's influence, Killua later retook it independently, eliminating all competitors to earn his license. His loyalty to Gon remained steadfast throughout their adventures.

Killua refined his combat skills through training in Nen, identified as a Transmuter specializing in converting aura into electricity. This created abilities like Thunderbolt and Godspeed, the latter granting extraordinary speed and automated defensive reflexes. His assassin techniques—stealth, poison resistance, and rhythmic evasion—complemented his Nen prowess. These skills were tested in battles such as the Heavens Arena tournament and conflicts with the Phantom Troupe in Yorknew City, where he protected allies despite the group's threat to Kurapika.

The Chimera Ant arc marked a critical turning point. Facing overwhelming opponents, Killua confronted deep-seated fears instilled by Illumi, particularly a paralyzing aversion to uncertain fights. Recognizing this fear endangered Gon during their mission, he physically extracted Illumi's control needle from his body. This act symbolized his rejection of conditioning and commitment to self-determination. His resolve was tested again when Gon suffered catastrophic injuries avenging their mentor Kite; Killua defied his family once more to rescue his sister Alluka, utilizing her wish-granting abilities to heal Gon.

Following these events, Killua prioritized Alluka's freedom, recognizing her suffering under the Zoldyck family's exploitation. He severed ties to travel with her, explicitly rejecting the assassin's path. His final interaction with Gon occurred atop the World Tree, where Gon acknowledged Killua's role before they parted ways. Killua's departure with Alluka underscored his evolution from a Zoldyck tool to a guardian forging his own purpose.

Appearances in spin-off films like *Phantom Rouge* and *The Last Mission* depicted Killua aiding allies against external threats. These stories did not alter his core trajectory, reinforcing his consistent abilities and loyalty as Gon's protector and a formidable combatant in standalone scenarios.