You have a Guardian Angel, but with a deadly hand-cannon instead of wings. Meet the Mysterious Stranger, an odd and eldritch entity said to appear and aid you in combat before you draw your dying breath. Of course, it helps to have the Mysterious Stranger Perk first...
The Mysterious Stranger in Fallout 3 and Fallout: New Vegas is heavily implied to be the father of The Lonesome Drifter. In speech, the Drifter will state that his father was a "mysterious" man, and that he was a "stranger" to his own family. The word mysterious appears right above stranger in the dialogue box. The Drifter carries a unique .44 magnum, which can be acquired by the player, that looks remarkably like the Mysterious Stranger's revolver; and when you draw your weapon, the same guitar notes that you hear when the Mysterious Stranger shows up are played, and another note is played when the weapon is holstered.
During the add-on Mothership Zeta, having the Mysterious Stranger perk opens up a unique dialogue option with Somah. "Maybe, if we're lucky, a friend of mine will show up to help us." To which she responds with confusion.
There have been cases of the stranger making movie like entrances and exits. An example is if near the subway escalators in front of Rivet City then he will walk up them and shoot the enemy.
ps3 If the Mysterious Stranger appears and there is a wall or other obstacle between him and his target, he will not take a shot. This will sometimes lead to you being stuck in V.A.T.S. permanently, necessitating a reload of an earlier save. [verified]
pc If the enemy is near a cliff and a weapon is used that knocks them off it without killing them, the stranger may spawn in the air off the cliff and fall to his "death" as a result. As he is an "essential" character he will be rendered unconscious and the "Mysterious Stranger is Unconscious" message will appear on screen like any other essential non-player character. This will not prevent him from appearing to aid the player at a later date. [verified]
pc You can sometimes knock the Mysterious Stranger unconscious if you attack a nearby enemy in V.A.T.S. with an explosive such as a grenade. [verified]