Number 48

One of the two mysterious armored guards defending Laboratory 5 from intrusion, Number 48 is also a victim of the facility's horrid human experiments involving the removal of souls from the bodies of nearby Central Prison's death row inmates. Before his official execution about two years before the start of the series, 48 was known about the country of Amestris as the infamous serial killer "Slicer" who, when finally captured, was discovered to have been a pair of murdering brothers. Like Alphonse Elric and Barry the Chopper, Slicer's soul was bound to an empty suit of armor by way of a Blood Rune, but unlike other known instances of this, 48's body contains two souls, with the older brother's soul bound by a blood rune in the armor's helm and the younger brother's soul bound by a blood rune in the armor's neckpiece.

Proficient with a katana, 48 is the guard who confronts Ed when the young state alchemist breaks into Lab 5 on a secret investigation and gives Fullmetal a bit of trouble when the head and body are separated. Though Ed is able to defeat the elder brother by way of his own cleverness, the battle fatigue and blood loss from the first part of the fight necessitates that Edward resort to Scar's method of partial transmutation to defeat the younger.

The Slicer Brothers, despite having been serial killers in adulthood and juvenile criminals as children, seem to have a sort code of honor as indicated by both brothers pointing out to Edward the location of their blood runes during combat. Though they no longer consider their lives human or of any value, they are amused when Edward continues to refer to them as "alive" and offer to answer some of his questions after their defeat.

Unfortunately, the brothers are killed by Lust and Envy before they can reveal very much information.

In the 2003 anime
In the first anime series, the younger brother takes his own life after being beaten by Ed, breaking his blood rune with his own hands. The elder explains that, after being called human by Edward, he feels content enough to move on rather than going back to his life under the Homunculi's control. The elder brother accompanies Edward, but is killed by Lust.