Mysterious Stranger (character)

The  is the individual that might appear if you have the Mysterious Stranger perk. He lends a helping hand in battles.

Background
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. The mysterious stranger perhaps originates from the famous pop-culture western film, The Good, The Bad, The Ugly, during the scene in which a gang attempts to capture a fugitive, and a mysterious individual suddenly appears, disposing of all several gang members with his revolver pistol. The individual wears the same clothing and weaponry as the mysterious stranger in the Fallout series.

Other interactions
The Mysterious Stranger will literally appear out of nowhere to assist you by finishing off your enemies. After the V.A.T.S. sequence finishes, he will disappear in a similarly abrupt manner.

Fallout Shelter

 * If one clicks on him they receive a cash bonus. This cash bonus is greater the closer he is to disappearing.
 * He arrives with his telltale theme music and leaves the same way. When he arrives and leaves the phone will vibrate.

Fallout 3, Fallout: New Vegas

 * A short guitar riff will play whenever the Stranger holsters or unholsters his weapon, similar to the Mysterious Magnum.

Fallout: New Vegas
He seems to have a son, The Lonesome Drifter, who wanders around the wasteland looking for his father.

Appearances
This appears in Fallout 3 and all its add-ons, Fallout: New Vegas and all its add-ons, Fallout 4 and all its add-ons, and Fallout Shelter.

Behind the scenes
The Mysterious Stranger is inspired by the trenchcoat-clad figures of 1950's film noir.

Fallout 3, Fallout: New Vegas

 * When you are too close to your target in V.A.T.S., sometimes the Mysterious Stranger will appear directly on your target and will miss all his shots.
 * 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.
 * 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. He may still continue to reappear in V.A.T.S. like normal, so no permanent damage seems to occur.
 * It has occurred that the Mysterious Stranger does not disappear upon exiting V.A.T.S., allowing you to "speak" with him and reverse pickpocket him for his outfit and hat. His dialogue is simply a blank box that goes away almost immediately. You can target him in V.A.T.S., which, amusingly, can cause another Mysterious Stranger to appear and attack the first one. Upon exiting V.A.T.S., they will both disappear.
 * At rare moments the Mysterious Stranger will spawn in the ground and literally shoot nothing.
 * The Mysterious Stranger will not function underwater. When the Stranger is activated underwater, the music will play, and the Stranger will swim around the target for a few seconds, then V.A.T.S. will exit. This has no negative effects other than the fact that an angry enemy is now chasing the player (which is a likely outcome when underwater since the player cannot be in sneak mode in such a circumstance), and the Stranger will continue to appear on a regular basis.
 * Sometimes, the Mysterious Stranger will spawn behind the player, who will be directly in the line of fire between him and the target. There is a good chance the player can be shot and likely killed, especially if it's in a doorway or someplace similar.
 * If the Mysterious Stranger appears when attacking a ceiling-mounted turret in V.A.T.S he will spawn inside the turret and miss all of his shots.
 * 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.