Reading Hunter X Hunter years ago we noticed a pattern in the Zoldyck children’s names.
Each brother’s name ends with the beginning of the another brothers name:
Irumi = Miruki = Kirua
イルミ = ミルキ = キルア
Karuto was missing a link to the pattern.
This was before I knew of Aruka, which fixes the link.
So that means, to fit the pattern, there must be a sixth brother:
Irumi = Miruki = Kirua = Aruka = Karuto = Torui?
イルミ = ミルキ = キルア = アルカ = カルト = トルイ?
I admit to not being up to speed on HxH, so this may already be old news, but I love nerding out on this stuff.