The Strange Man:

The strange man is featured in both Red Dead Redemption and Red Dead Redemption 2 as a stranger for whom the player may interface with, unlocking a series of side missions or separate encounters.

Background:

An enigmatic and otherworldly character, the Strange Man appears to know John Marston very well, including details about his past life in the Van der Linde gang that not even John himself remembers.

Mission Appearences:

Theorized Partial Residence: Bayall Edge in Bayou Nwa, State of Lemoyne (RDR 2) Transitory (RDR)

Events of Red Dead Redemption 2:

There is an abandoned shack in Bayall Edge, Lemoyne that seems to belong to the Strange Man. The shack is riddled with cryptic writing alluding to Armadillo, Herbert Moon and the cholera outbreak of New Austin. If the player visits it as Arthur Morgan, there will be an unfinished painting of the Strange Man. If the player visits the shack as John Marston, the painting will gradually become more and more complete over the next few days, and John will document his thoughts of it in his journal, saying how it makes him feel "fascinating, awful and seductive, all at once". John can also see the Strange Man stalking him in the mirror like a phantom.

A stranger in New Austin, around Hennigan's Stead, alludes to the Strange Man. He states how a man wearing a "black suit and big top hat," whom he believes to be the Grim Reaper, haunts and curses the town of Armadillo, being the reason for the Cholera epidemic. Additionally, Herbert Moon has a portrait of the Strange Man in his general store, similar to the one in Bayall Edge, though much smaller. John will notice and asks about it, claiming he recognizes the man, but Herbert Moon shrugs it off, saying that somebody gave it to him and that he has it in his store because he likes it.

Events of Red Dead Redemption:

The Strange Man is the focus of the stranger side-mission "I Know You", which is only available for John Marston. John meets the Strange Man in three separate places during the mission, once in each territory of the game.

Each time, the man gives vague answers to John's questions, and twice, asks John to perform errands for strangers. The first is to visit a man about to cheat on his wife, the second is to visit a nun trying to raise money for charity. The player can either make the morally good choices things of stopping the man and donating to the nun respectively, or the morally wrong choices encouraging the man to cheat on his wife and robbing the nun.

The Strange Man appears one final time overlooking Beecher's Hope. He is unforthcoming about who he is, and his standoffish demeanor enrages John enough to open fire on him. The Strange Man is completely unaffected and mysteriously vanishes right before John's eyes.