Well, They only appear to people that truly need a change in their life. People down on their luck, but still do the right thing. Keiichi is a perfect example of this. He had terrible luck his whole life, people treated him poorly, he had no girl friend, but he still helped a little girl find her mothers wallet when he was already late for something else. That is why they choose to let him have 1 wish. His wish was a bit... different then most people. "I wish for a goddess like you to stay by me forever." That's how he meet Belldandy, and life has been exciting ever since then.

So this Mario guy must be a great hero from your world.