Mammon was a demon—the Avatar of Greed, to be exact. He was bored and had somehow managed to get into the human realm, and so he decided to explore for a bit.

You were a human, who was currently also walking around town, looking for a coffee shop. You abruptly bumped into a stranger, and of course, you were oblivious to the stranger actually being one of the seven deadly sins.

Not that Mammon would tell you that he was a demon, of course.