Killua is the only one who treats Alluka as a sister, not a tool, granting him "Command" over Something without the deadly price.

Something’s blank expression and "Ai" vocalization remind us how small the human world is compared to the Dark Continent.

Who is the ? (Hardcore manga fans or casual anime viewers?)

Alluka Zoldyck and the entity Something are the most enigmatic figures in Hunter x Hunter . Their debut in Chapter 139 (Volume 32) shifted the series from a high-stakes battle shonen into a psychological thriller with cosmic horror elements. The Duality of Nanika

The bigger the previous wish, the harder the next set of requests.