Alluka Zoldyck

Alluka Zoldyck is the missing child of the Zoldycks and has only appeared once during the series. In Chapter 229 of the manga, his back is shown in a picture of the five Zoldyck sons along with their mother. His name is officially stated in the official Hunter × Hunter Guide book in a family tree of the Zoldycks. An alternate way of discovering his name is via a pattern seen with the Zoldyck sons from oldest to youngest: IlluMilluKilluAlluKalluto. It is speculated he left his family and that his younger brother Kalluto has joined the Phantom Troupe in order to find him.