Slicer Brothers

The Slicer Brothers are a pair of serial killers who become one of the guardians of Laboratory 5 under the joint alias of Number 48. On the day they were supposed to be executed, they instead became victims of the the fifth laboratory's horrid human experiments involving the removal of souls from the bodies of nearby Central Prison's death row inmates. They had their souls bound to a single suit of armor by way of a pair of blood runes (one in the helm and one in the armor's neckpiece) and were assigned to kill any intruders at the laboratory. Prior to their official "execution" about two years before the start of the series, they were 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.

The younger brother, whose soul resides in the suit of armor's body, is proficient with a katana. The brothers, as Number 48, confront Edward Elric when the young state alchemist breaks into Laboratory 5 on a secret investigation. Initially, Edward believes that there is only one soul in the armor, as he only sees the older brother's blood rune, located in the helmet. When he is finally able to separate the helmet from the rest of the armor, he thinks he has won the fight, is surprised to see the body keep on fighting on its own. By this point, the battle fatigue, blood loss, and his right arm unable to move (which he believed to be from damage, however it is revealed that Winry just forgot to put an important screw into the automail) from the first part of the fight necessitates that Edward resort to Scar's method of partial transmutation to defeat the younger brother.

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 eventually pointing out to Edward the location of their blood runes during combat. They also wish for Edward Elric to take their lives after being defeated, but Edward refuses to kill them on the grounds that they are living human beings. Though they no longer consider their lives human or of any value, they are amused when Edward continues to refer to them as living human beings, and offer to answer some of his questions.

In the manga and 2009 anime
Not much differences in appearence except for the armor being green rather than the usual red. Unfortunately, before they are able to reveal very much information to Edward, the brothers are killed by Lust and Envy.

In the 2003 anime
After being defeated, the younger brother makes up his mind and takes his own life by breaking his blood rune with his own hands, saying to his older brother that he is "going ahead" and that he will be waiting for his brother in the afterlife. 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 older brother lives long enough to take Edward to where the Philosopher's Stone is being developed before Lust slowly kills him in an attempt to force Edward to help her.

Trivia

 * In a humorous manga omake, it's revealed that the older brother loves cats and, much like Alphonse, tends to hide them in his suit of armor. In contrast, the younger brother likes dogs and often gets into arguments with his brother over which is better - cats or dogs.
 * In the manga, Slicer Brothers' Blood Runes use hexagrams as a part of design. They were altered in the second anime series to avoid possible reference to Judaism.