Another name for the mythical, special someone who helps the hero on his/her journey is:

The mysterious stranger.

Explanation:

In 'Myth and The Origin of the Humanities', the author narrates the origin of myth and how it gave birth to humanities.

In myths, when a hero/heroin are on their quest, and gets into trouble or are about to loose their last breath, a mythical character seems to appear from nowhere and disappear back to unknown. In literature, this mythical character is known as 'special someone', who helps hero/heroin on their   quest.

The another name used for this mythical special someone is mysterious stranger. This character comes from nowhere and helps the hero/heroin and then goes back to the unknown, creating mystery and is unknown to anyone.

Therefore, mysterious stranger is the correct answer.