He travels through time with a large backpack filled with all his worldly possessions. He's a thin, slightly worn-out looking man, who isn't actually a stranger to Winden: He's grown-up Jonas Kahnwald.
The Stranger seems disillusioned and full of regrets. He is working tirelessly to undo some of the unfortunate events in Winden that he has helped to create. While he suspects he can't actually change the course of time, he hasn't given up hope. For a long time he works alongside Claudia Tiedemann in her fight against the Sic Mundus cult and their leader Adam.
The Stranger and Jonas
Jonas and the Stranger meet each other several times in the days following Mikkel Nielsen's disappearance. In 1986, the Stranger warns Jonas not to take Mikkel back to 2019 lest he prevent his own birth.
He sends Jonas the goodbye letter from his father and a Geiger counter that helps him find his way through the caves to the passage. When Jonas is locked in the bunker by Noah and Helge Doppler, the Stranger shows up again, revealing his true identity, but not letting him out.
The Stranger's plan is to close the time travel passage in the Winden caves – the passage that Mikkel and others traveled through. To do this, he needs to get his time machine repaired by the clockmaker and supply the radioactive Caesium-137 isotope. He gets his hands on the substance from the yellow barrels that have been removed from the nuclear power plant site and stored at a truck stop. He manages to activate his machine in the caves, thus closing the passage through time.