Mysterious Stranger (Fallout)

The Mysterious Stranger is the individual that

Background
Once acquired he lends a helping hand in combat.

Other interactions
There is a chance that Mysterious Stranger will appear as a temporary ally during random encounters.

Chance of appearance %= 50 (Fallout)
 * $$\text{Chance of appearance }%=30+(2\times\text{Luck})$$ (Fallout 2)

Other interactions
The Mysterious Stranger will sometimes appear with you on random encounters. He stays on the map after combat ends.

Appearances
This Mysterious Stranger appears only in Fallout.