Illumi Zoldyck

As the eldest sibling of Killua, this seasoned assassin played a pivotal role in his brother's brutal upbringing alongside their father. Known for his terrifyingly oppressive aura, he even utilized specialized needles to manipulate his facial structure and hide his identity during the Hunter's Exam. His deep, unsettling influence over Killua is reinforced by a hidden needle implanted in his brother's brain, instilling a profound sense of dread and involuntary obedience whenever the two encounter one another.