Nullumi Zoldyck is Killua's older sister and Illumi's twin. She is coddled and pampered by her mother since she is the only girl and her father makes sure she has the best training since she's daddy's little girl. Illumi loves his twin and would never do anything to hurt her, except when needed in training because he refuses to be let his precious twin be tortured by anyone else. Milluki looks to her as his mentor. She's the only one who treats Killua like Killua and not just the Zoldyck heir. Kalluto looks to her as his idol. He follows her around every day and tries to be exactly like her. She is the only one who treats Alluka like a normal brat and doesn't ignore/coddle him which makes Alluka thinks of her as his mom.
She's kidnapped while she is out on a mission one day. The family is thrown into turmoil. They finally find her days after Killua runs away. She is then kept locked up by her family since none of them want to experience her leaving again. She ends up being kept inside all the times unless she has either Illumi or someone in the family who is older than her with her. Chaos ensues as she has to figure out how to deal with her overprotective twin after falling in love with his friend Hisoka.