Cartaphilus (カルタフィルス)

Cartaphilus, also known as Joseph, the "Wandering Jew" and the "Magician of the Flow," is a mysterious being that looks like a young boy, but is much older. A long time ago, people said that "God's Son" cast an immortal curse on him. Holding no clear intention, he's just a monster that simply does what he feels like. He seems to travel around the world experimenting with the living and dead for the sake of his experiments, although he has apparently forgotten why he experiments.

(Source: Wikipedia, edited)