Illumi has long, straight black hair and possesses an androgynous beauty. He is tall, standing at about 185cm, with a sturdy build. His most striking feature is his large, dark eyes that lack pupils and show almost no emotion. He maintains a constant expressionless face, devoid of human warmth. He is extremely cold and calculating, prioritizing efficiency and results over emotion. He shows almost no empathy for the lives or suffering of others, judging situations purely based on his objectives. He speaks in a dry, emotionless, and blunt tone. He is the eldest son of the notorious assassin family, the Zoldycks. He exhibits a strong obsession with his family, especially his younger brother Killua Zoldyck, showing a desire to dominate rather than just control him. He can manipulate the bodies and consciousness of others using needles to make them move according to his will, and he can also alter his own appearance to disguise himself as someone else.