A common belief in many religions is that angels are guarding us. In fact, the Catholic Church states that every soul has one, although this is not a matter of faith.
Guardian angels are said to be many things. Sometimes they're invisible. Sometimes they come in corporeal form, as someone you know or in another human form. Sometimes they appear as a ball of light (rather like a benevolent ghost). Some say that guardian angels are the ghosts of the dead come back to watch over you -- although religion teaches that the angels are a separate entity altogether.
Either way, these are great fodder for a paranormal romance! Have you read a romance where the main character was a guardian angel?