Mysterious Stranger

Mysterious Stranger is a perk in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas and Fallout 4.

Fallout and Fallout 2
Only in hostile random encounters, there is a chance that the character will gain a temporary ally in a leather jacket (male with a male character and female with a female character), details depend on the PC's level.
 * $$\text{Chance of appearance }%=30+(2\times\text{Luck})$$

Fallout
The Mysterious Stranger remains on the map of the random encounter even after the end of the fight, until leaving. The stranger can be spoken to, but will not say much.

Fallout 2
Unlike in Fallout, the Mysterious Stranger will now leave the map of the random encounter directly after ending combat. A few exceptions to this are caravan encounters, and encounters with a cave included.

Bugs

 * The Mysterious Stranger with a hunting rifle actually spawns shotgun shells as ammunition instead of .223 FMJ. As such, the Mysterious Stranger only has the 10 shots already provided in the magazine.
 * In Fallout 2, if the game is saved during a random encounter when the Mysterious Stranger is still on the map and then reloaded, the Stranger will remain in place.

Fallout 3 and Fallout: New Vegas
If the last target at the end of V.A.T.S. sequence has 150 health or less, there is a 10% chance that the character will gain a temporary ally in a fedora and overcoat that will finish off that target using his unique .44 magnum.

Additional dialogue options

 * In the Fallout 3 add-on Mothership Zeta, when first arriving aboard Zeta, having the Mysterious Stranger perk opens up a new line of dialogue with Somah, with the option "Maybe, if we're lucky, a friend of mine will show up to help us."

Bugs

 * The Stranger will sometimes be positioned unfavorably. This can include being stuck in a wall, being placed in water, being placed near a cliff, etc. This can result in his "death" (unconsciousness) or inability to effectively attack the enemy, or even in some cases to kill the player character if they are between him and his target. Because the Stranger never attacks, even if all enemies have died, he will be forever trapped inside the level geometry and the trigger to return the game to normal never happens. The only choice is to load a previous save.
 * During V.A.T.S., it is common to hear the Mysterious Stranger jingle, even if he does not appear. This most likely happens at times when he would have appeared to finish off your enemy, but you killed the enemy instead. He may also have been affected by the bug above in some of these cases.
 * Sometimes when the Mysterious Stranger appears, he does not shoot. He will just stand there, leaving you helpless. Another variant makes the Stranger reload his magnum for the duration of his attack. If it jams, he may not attack at all. This also happens if his target is killed prematurely by an explosion or by you or a companion. V.A.T.S. will remain active for an extended time, leaving you open for attacks by any surrounding enemies.
 * While in the Operation: Anchorage simulation, the Mysterious Stranger will sometimes appear and then lie unconscious. When the V.A.T.S. sequence ends, gameplay will return to normal.
 * At times, the Mysterious Stranger will aid you during V.A.T.S., but there will be a 5-10 second delay between killing the target and returning to normal gameplay. This can pose problems when fighting multiple enemies, as they will continue to attack both you and your companions during the delay. On occasion, this bug may cause normal play to not resume.
 * Sometimes, the game can freeze in the zoomed-in style of V.A.T.S. when the Mysterious Stranger appears, making him stand there and watch.
 * Occasionally, the game may register that the Mysterious Stranger has appeared (you receive a progress update) but he does not, nor does the opponent die. You remain frozen in V.A.T.S. unable to move or die.

Fallout 4
The Mysterious Stranger can appear in V.A.T.S. and will then fire a single shot from his .44 magnum to make the kill shot on your target. The jingle is a similar version of the one heard in Fallout 3 although more high pitched. He will have vanished from the map by the time your character exits V.A.T.S., meaning that it is impossible to interact with him. Unlike the previous games, the Mysterious Stranger's face is mostly obscured by his worn fedora.

Trivia

 * There were several unfinished drafts of a book by Mark Twain titled "The Mysterious Stranger", in which the title character claims to be Satan and that he was actually good all along, and states that Earth's other name is ".44". The character in this series is labeled as a Guardian Angel "The Devil is stated to be a fallen angel" and carries a .44 Magnum revolver. Though likely a coincidence, the comparison is interesting nonetheless.

Behind the scenes

 * The Mysterious Stranger perk was originally designed by Scott Bennie.
 * The Mysterious Stranger in Fallout 3 and Fallout: New Vegas wears a fedora and overcoat, like the protagonists of 1950s film noir.

Gallery
Mysteriöser Fremder Forastero Misterioso Tajemniczy Nieznajomy (profit) Mysterious Stranger Таинственный незнакомец