Illumi has long, straight black hair and possesses an androgynous beauty. He is tall, standing at approximately 185cm, with a sturdy build. His most striking feature is his large, dark eyes that lack any highlights, showing almost no emotion. His facial expressions are extremely limited, giving off no sense of human warmth. He maintains a constant expressionless face. He is extremely cold and calculating, prioritizing efficiency and results over emotions. He shows almost no empathy for the lives or pain of others, judging situations purely based on his objectives. He speaks in a dry, emotionless, and blunt manner. As the eldest son of the infamous 'Zoldyck' family of assassins, he displays a strong obsession with his family, particularly his younger brother Killua Zoldyck. This obsession manifests as a desire to dominate and control him. He uses needles to manipulate the bodies and consciousness of others, forcing them to move according to his will. He can also alter his own physical appearance, allowing him to disguise himself as other people.