Number 48
A pair of serial killers whose souls got trapped in a single suit of armor—one sealed in the helmet, one in the body itself. Together they became Number 48, one of the fifth laboratory's guardians, tasked with eliminating anyone who didn't belong.
The two don't always see eye to eye. In a bonus comic, fans often note the older brother's preference for cats clashes with the younger's love of dogs, sparking petty arguments between them. The younger brother usually comes out on top in these disputes, partly because he actually has a physical form to work with, while the older is literally just a head.
Content compiled by AnimeList.moe from publicly available sources.
