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.

History
On the day they were supposed to be executed, they instead became victims of 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 neck piece) 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, but is surprised to see that the body keeps on fighting on its own. By this point, the battle fatigue, blood loss, and his right arm unable to move 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
Ultimately, they have a much smaller role in the original story, being killed by Lust and Envy after their fight before being able to reveal too much information to Edward.

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.

Abilities
Whilst the older brothers abilities are left in question, the younger brother's abilities are shown during the bout with Edward. The younger brother is extremely skilled with a katana, to the point where he easily kept the upper hand for the entirety of the fight and forcing the latter almost completely on the defensive.

Though, it still remains unclear how much influence the older brother really had over the armor, as, when the armor was connected to the head, both brothers could, theoretically, have been in control.

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. These arguments are often one-sided, as the younger brother controls most of the body, so the older can do little in defense.
 * In the manga, Slicer Brothers' Blood Runes use hexagrams as a part of design. They were altered in both anime series to avoid possible reference to Judaism.
 * In the 2003 anime, the Brothers' armor is red, while in the 2009 anime, it's green.
 * In one Omake a picture is shown of a combination between Barry the Chopper, Slicer, and Alphonse. Above it is the words "SUPER COMBINATION".
 * Even though they both die early on, half of the elder brother's helmet does make a cameo towards the end of the story when the five Transmutation Circles in each lab are activated, though this is manga only; it was not included in the 2009 series.
 * Due to the manga later establishing that a human body and soul must be alive for either to continue existing, it can then be assumed that both brothers' bodies were indeed alive somewhere else without their knowledge, presumably in a situation similar to Barry the Chopper's body.