The Postman (Hans) is one of the NPCs of NieR Replicant. He runs the Post Office in Seafront and plays a major role in a number of Side Quests. There is only one Postman and Post Office, but the Postman's Apprentice can also be seen standing around Seafront.
Post Office Back Room
When visiting the Post Office, there is an accessible backroom with a box moving puzzle. This will not lead to anything unless the specific quest related to the Post Office has been started. (See: The Lighthouse Lady's Wrath)
The Postman is a client for a number of quests that mostly also involve the Lighthouse Lady:
Quest Spoilers Ahead: The Postman's name is Hans, though it is not mentioned anywhere in the game. NieR starts off his time in Seafront with quite a negative impression of the Lighthouse Lady and she seems to be infamous among the residents. Of all the residents who know her well, it would be the Postman. One of his first tasks for NieR was to deliver a letter to the Lighthouse Lady for the Postman. The other postmen didn't want to deal with her cranky behavior.
Despite the first impressions, NieR later finds out that the Postman actually cares for the old Lady. He uncovers a town secret that with their combined efforts, they have been secretly writing her letters signed from her far foreign lover who has actually long passed away. To keep her hopeful, the Postman's father and the other residents kept the news from her and instead, wrote on his behalf for the past 50 years until the task was passed on to his son, the Postman.
When NieR finds out about the secret, the Postman shares the stack of unsent letters from the Lighthouse Lady and leaves the decision to tell her the truth to NieR.