Hans Weiss is a medic in the German army. He appears in non-player roles in Chapter 1 and 4 of the game. His backstory and connection to Walt is expanded on in the Valiant Hearts: Dogs of War comic.
Hans first appears as Walt's handler in Chapter 1 at Neueve-Chappelle. After Emile pours Walt water, Hans moves to smoke tobacco in his Calabash smoking pipe. He does not appear again until Chapter 4 when Karl and Walt meet him in occupied Saint-Mihiel. Soon after the joyous reunion with Walt, Hans dies when the town is attacked.
In the Valiant Hearts game, Hans has a role as a supporting character only, and no information is given about him in the game or any of the official character profiles, beyond the fact that he is a German medic connected to Walt.
In Valiant Hearts: Dogs of War
Hans is included in the iOS comic about Walt's backstory. His story is expanded and filled out along with Walt's, and his occupation and surname are revealed. He is a doctor from Cologne, and is one of the few characters in Valiant Hearts with a stated surname.
In the comic, Hans is a friend of Walt's breeder, Anton. In 1911, Walt helped to save a soldier from drowning in a river near Anton's farm; Hans was visiting at the time and was on the scene to give the man medical attention. After that, he quickly made friends with Walt. Because Walt was given to the military only a few days after, Hans did not meet him again until a few years later.
In 1914, Hans was drafted to serve in the military. When he arrived at camp, he was reunited with Walt, who was being trained as a medical dog. Two months later, Hans was assigned to the casualty unit, and Walt was the dog paired with him.
Hans and Walt stayed together until they became separated at Neueve-Chappelle in early March of 1915.