Only in hostile random encounters, there is a chance that your character will gain a temporary ally in leather jacket (male with a male character and female with a female character), details depend on the player's level.
The Mysterious Stranger remains on the map of the random encounter even after the end of the fight, until you leave. You can start a conversation with them but they will not say much.
If you attack them, the Mysterious Stranger will become hostile toward you; however, they will become friendly during the next hostile random encounter.
The inventory of the Mysterious Stranger is replaced at each random encounter.
If the Mysterious Stranger ever dies, they will no longer spawn.
The Mysterious Stranger perk in Fallout is stronger than in Fallout 2 since the Mysterious Stranger is already at their maximum statistics and their plasma rifle is more powerful in Fallout than the H&K G11 that the Mysterious Stranger carries in Fallout 2.
pc The Mysterious Stranger with the 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 clip. [verified]
pc In Fallout 2, if you save during a random encounter when the Mysterious Stranger is still on the map and then load that save they will remain frozen in place. [verified]
Fallout 3 and Fallout: New Vegas
Gives you your own personal guardian angel... armed with a fully loaded .44 magnum. With this perk, the Mysterious Stranger appears occasionally in V.A.T.S. mode to lend a hand, with deadly efficiency.
Only in V.A.T.S., there is a flat 10% chance that your character will gain a temporary ally in fedora and overcoat armed with a powerful and unique .44 magnum, dealing exactly 18002 (9001 + 9001 critical damage with 100% critical multiplier) damage per shot and will kill any enemy in one hit. Regardless, he will still empty six shots into a chosen target.
Additional dialogue options
In the Fallout 3add-onMothership Zeta, when you first arrive aboard Zeta, having the Mysterious Stranger perk opens up a new line of dialogue with Somah, with you saying "maybe, if we're lucky, a friend of mine will show up to help us."
In Fallout 3, be aware that "mesmerizing" non-player characters with a Mesmetron in V.A.T.S. will sometimes make the Mysterious Stranger appear, resulting in the death of your victim.
The Mysterious Stranger will sometimes seek out other hostiles while in V.A.T.S., after he has killed the one you were targeting.
His appearances have a counter called "Mysterious Stranger Visits" that is displayed randomly during the loading screens of Fallout 3, and on the Pip-Boy 3000 under the "Stats tab - General" in both Fallout 3 and Fallout: New Vegas.
In Fallout: New Vegas, the Miss Fortune perk functions in the same way as this perk, but with different effects in combat.
Both the Mysterious Stranger and Miss Fortune perks can be chosen by the player, but they will not appear together in the same V.A.T.S. sequence.
When interrogating Silus at Camp McCarran, if he is being attacked by the Courier using V.A.T.S., Silus will be killed by the Mysterious Stranger should he appear. In this event, Carrie Boyd will not respond in any way to Silus' death.
It is possible to attack and even cripple the Mysterious Stranger by accident in V.A.T.S, but this seems to have no effect whatsoever.
The Mysterious Stranger can be incapacitated by nearby explosions, sending him flailing about.
If the Mysterious Stranger/Miss Fortune appears in gas-filled area, their gunfire will accidentally ignite the gas and hurt the player and enemy alike.
In Fallout: New Vegas, after the Mysterious Stranger or Miss Fortune appears 15 times, the Friendly Help challenge is completed, increasing the probability for them to appear in combat much more often.
xbox360ps3pc The Stranger will sometimes be positioned unfavorably. This can include being stuck in a wall or such, 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 if the player is between the Stranger and his target. NOTE THAT THIS WILL RESULT IN YOUR GAME ENDING. 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. Your only choice is to load a previous save. [verified]
xbox360 During V.A.T.S., it isn't uncommon to hear the Mysterious Stranger music, 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. [verified]
xbox360ps3pc Sometimes when the Mysterious Stranger appears, he doesn't shoot. He'll just stand there, leaving you helpless. Another variant causes the Stranger to 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. [verified]
ps3xbox360 While in the Operation: Anchoragesimulation, the Mysterious Stranger will sometimes appear and then lie unconscious. When the V.A.T.S. sequence ends, gameplay will resume to normal. [verified]
xbox360ps3pc 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. [verified]
xbox360ps3pc 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. [verified]
xbox360 Occasionally, the game may register that the Mysterious Stranger has appeared (you receive a progress update) but he does not appear, nor does the opponent die. You remain frozen in V.A.T.S. unable to move or die. [verified]
Behind the scenes
The Mysterious Stranger perk was originally designed by Scott Bennie.