I Know You

 is a side-mission in Red Dead Redemption.

Story
You encountered a strange man who seems to know a lot about your past and claims to know you from your days running with Dutch van der Linde. He asked you to go to Thieves Landing to check on a friend of his, who is drunk and thinking of cheating on his wife. The man did not explicitly say to stop him, just to advise him on how to best proceed.

Whether you stop the friend or let him continue comes up in your next meeting with the Strange Man, although it does not effect the outcome of the second encounter.

You meet the Strange Man again in Mexico, where he suggests you either help a nun who is collecting donations in a dangerous area, or just rob her yourself.

The third and final confrontation with the Strange Man takes place outside Beechers Hope, where after more enigmatic dialog, such as him not knowing his own name, collecting accounts of a sort, and being damned by many people. John loses his temper after asking the Strange Man for his name and fires on him, but the man is not effected by the bullets and walks away.

The Strange Man can be seen in the back of the firing line that kills John in The Last Enemy That Shall Be Destroyed.

It should also be noted that once you're playing as Jack, you cannot complete this stranger mission. It is not yet confirmed if this will prevent getting 100%.

Walkthrough
After meeting the Strange Man, head to Thieves Landing and go to the saloon. The drunk friend should be on the porch. You can either dissuade the drunk friend from cheating on his wife, or give him $15 to persuade him to cheat on his wife. Giving him the money will cause you to lose Honor.

After the Fort Mercer missions, he can be found in Mexico, giving you the choice of donating $10 to, or robbing, a nun.

There is one last interaction with the Strange Man, on top of the hill overlooking the Marston Ranch.

Trivia

 * The Strange Man could be interpreted to be God. A literal example of this is the Mexican nun, who says God sent Marston to her. A non-literal example can be inferred from the Strange Man's dialogue after Marston damns him: "... and I've been damned by countless people..." as a reference to the phrase "god damn it". This also seems to fit with his thorough knowledge of John and his non-reaction to being shot.
 * The Strange Man wears a slick, black suit and mustache which are more common descriptions of the Devil. Not only that, but the entire time he is judging your morality, which is more a trait of the Devil. Also, "god damn it" does not mean "to damn god". The Devil was damned to hell by God and when something goes wrong, people generally blame or damn him.
 * You can't do this mission after you've finished the game. The NPC can't be found anymore.
 * The Strange Man is standing in the exact spot that John Marston is buried at later in the game.
 * The Man has also been interpreted as a personification of John's own conscience or of his father.