Actually the Norse God of thunder, Thor, in the form of a teenage boy with brown hair and eyes. He was sent to Earth from the realm of gods to kill Loki; in the manga, he doesn't remember this at first, while in the anime, he chooses not to kill Loki because he sees no reason to. Unfortunately, he forgot how to return to the realm of the gods, so he is forced to work a wide array of part-time jobs to support himself, which becomes another one of the series' recurring jokes. He always carries around a wooden sword, which is actually Thor's hammer Mjollnir in disguise.

(Source: Wikipedia)