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Killua Zoldyck is one of the main characters in the anime series, HUNTERxHUNTER. He first meets Gon Freecss during the Hunter Exams and instantly became best friends. Being part of the Zoldyck family, Killua was raised to be a master assassin and heir to the family name, but decided to run away and explore the world. After meeting Gon, Killua realizes there is more fun things in life to do with his best friend.

Hunter × Hunter (ハンター×ハンター, Hantā Hantā), pronounced "Hunter Hunter", is a Japanese manga series written and illustrated by Yoshihiro Togashi with two anime adaptations. The first chapter of the Hunter × Hunter manga series was published in March 1998, in Weekly Shōnen Jump.

The story of Hunter × Hunter focuses on a young boy named Gon Freecss, who one day discovers that the father he has always been told was dead is alive and well. He learns that his father, Ging Freecss, is a legendary Hunter. Despite the fact that his father abandoned Gon with his relatives in order to pursue his dreams, Gon becomes determined to follow his father's footsteps, pass the Hunter Exam, and eventually find his father to prove himself as a Hunter in his own right.

(from Fandom Hunter x Hunter Wiki)