Killua Zoldyck is characterized by his spiky silver hair, pale skin, and sharp blue eyes that change shape depending on his emotional state. When he enters his “assassination mode,” his eyes narrow and take on a more intense, sharper appearance. At the beginning of the series, Killua is quite lean due to the rigorous physical training and torture he endured throughout his childhood as part of his assassin upbringing. Over time, as he grows older and stronger, his physique becomes more muscular and toned.

His usual attire consists of baggy, comfortable clothing. Typically, he is seen wearing long-sleeved shirts, often dark-colored and with turtlenecks. His shorts are notably long and loose in the manga and the 2011 anime adaptation, but in the Nippon Animation version, his shorts were shortened to just above his knees. Killua also wears purple boots, which were recolored as brown and black in the Nippon Animation adaptation.

As a young child, Killua’s hair was longer, almost reaching his shoulders. His outfit during his childhood included a blue hoodie, grey pants, and shoes, reflecting the more casual and practical clothing style he favors throughout his appearances.

Killua Zoldyck is initially introduced as a mischievous, cheeky, and cheerful character who shares a similar playful nature with his best friend, Gon Freecss. However, while Gon is polite and kind to most, Killua can be quite rude, especially toward strangers and older individuals. His rebellious attitude is often offset by his well-known sweet tooth, with a particular fondness for chocolate. InHeavens Arena, he once spent nearly 200 million Jenny on snacks, with his favorite being Chocolate Balls and Chocolate Robots.

One of Killua’s defining traits is his quick thinking and analytical nature. He is often able to assess situations with a level of ease and precision that surpasses most of his peers, giving him the attitude of someone much older. Early in the series, he displays confidence in his abilities, even questioning Gon’s potential during the Hunter Exam. However, as time goes on, he becomes more humble, no longer boasting about his strength. Despite his calm and pragmatic nature, Killua is often embarrassed when Gon expresses their deep friendship or compliments him openly.

Despite the brutal upbringing and torturous training Killua endured at the hands of his assassin family, he manages to retain a noble and pure side. His ability to smile, despite the traumatic experiences of his childhood, is noted by both Netero and Biscuit as being remarkable. Killua’s Nen ability to manipulate electricity further emphasizes the tragic nature of his past, with Biscuit commenting that it’s a wonder Killua can still smile after such suffering.

However, Killua’s assassin instincts remain strong, and his deadly, bloodthirsty nature surfaces when provoked. Though his time spent with Gon softens him, he is still willing to kill anyone who threatens his friends or family, particularly Gon or his sister, Alluka. This conflict between his killer instinct and his innate kindness creates a complex personality, as Killua’s loyalty to his friends often pushes him to sacrifice his own well-being. He is fiercely protective and will go to great lengths to ensure their safety, often without concern for his own health or life.

One of Killua’s most significant internal struggles is his fear of those more powerful than him, particularly his older brother, Illumi. Trained by Illumi to only engage in battles he can win, Killua is taught to retreat at the slightest sign of disadvantage. This mindset hinders his growth as a Hunter, leading Biscuit to predict that Killua would one day abandon Gon due to this fear. Killua’s journey of overcoming this fear is a central part of his character arc. During an encounter with a Chimera Ant, Killua discovers that Illumi had implanted a needle in his brain, which had been controlling his actions, forcing him to retreat whenever danger arose. Upon removing the needle, Killua is finally freed from this mental restraint and gains full control over his decisions, allowing him to fight without hesitation when necessary.

Within his family, Killua holds mixed feelings. He respects his father, Silva, and his grandfather, Zeno, though he finds his mother, Kikyo, to be annoying. Despite the constant death threats from his brother Milluki, Killua maintains a relatively “normal” sibling relationship with him, occasionally helping him when necessary. Killua’s greatest conflict remains with Illumi, whose influence he eventually breaks free from, but beyond that, he does not harbor hatred or fear toward the rest of his family.

Killua Zoldyck was born into the infamous Zoldyck family, a lineage of highly skilled assassins. From birth, he exhibited extraordinary potential, and as part of the family tradition, Killua underwent rigorous assassin training under the guidance of his father, Silva, and older brother, Illumi. By a young age, Killua had already mastered various deadly techniques and was considered to be one of the most promising assassins in Zoldyck history. His training began early; at the age of three, he was already engaging in life-threatening exercises such as tailing dangerous targets.

At the age of six, Killua was forced by his father to participate in the martial arts tournaments at Heavens Arena, an arena known for hosting deadly battles. He was required to fight until he reached the 200th floor of the tower, a milestone that signified a highly skilled fighter. It took him two years to reach this level, having reached the 150th floor within two months. His time in the Heavens Arena honed his already impressive combat abilities.

During this time, Killua also developed an aptitude for various skills that went beyond combat. By the age of six or seven, he had mastered the game of darts as part of his extensive training. Around the age of nine, Killua encountered Canary, a newly hired servant of the Zoldyck family, and attempted to befriend her. Although she was touched by his gesture, Canary declined his offer due to their differing social statuses.

However, despite his remarkable abilities, the Zoldyck family deemed that Killua lacked the cold-hearted mindset required to become the perfect assassin. To address this, Illumi implanted a special needle in Killua’s brain that subliminally altered his behavior, making him retreat whenever he sensed significant danger. The needle also served to prevent Killua from attempting to free his sister, Alluka, from her isolated playroom.

As a young boy, Killua became disillusioned with the life of an assassin. About three years before the events of theYorknew Cityarc, his father ordered him and his brothers to avoid the Phantom Troupe after completing a high-profile assassination of a Troupe member, an event that further disillusioned Killua. At the age of eleven, tired of the constant killing and control, Killua rebelled against his family, injuring both his mother and older brother before running away. He sought out the Hunter Exam for a new sense of purpose and adventure, taking the exam on a whim. By this point, Killua had already killed more people than the infamous Bombers combined, a testament to his deadly skill even at such a young age.