Hunter x Hunter anime. Killua. Kun.

Hunter x Hunter is a popular anime centered around the adventures of a boy named Gon Freecss, who aims to become a professional "Hunter" like his father. Throughout his journey, he meets unique friends, the most notable being Killua Zoldyck, a boy from a family famous for assassinations, known for his intelligence and extraordinary speed.

As for "Kun", it is a suffix used in the Japanese language to refer to boys in a friendly or informal way, usually among friends or when addressing someone younger.