Brave Story/WMG

Wataru is the Southern United Nations, Mitsuru is the Northern Empire.
The two of them were unusually in each other's Visions, so both of their individual visions were literally patched together to get the one we see. This is supported by various things - such as personal affinity. Not to mention that Mitsuru only finally meets a double of someone he knows in the Nothern Empire.

Vision is a variation of the Enderverse's Fantasy Game
Vision is created from the minds of the Travelers. As explained in the novel, no two Travelers have the same Vision.

Vision is a small piece of Fantasia/Fantastica
Similarly, the Goddess of Fate is also the Childlike Empress.

Miina/Meena is Vision's representation of Mitsuru
Both lost their families and are swayed into doing some rather unkind things in an effort to get them back. Miina/Meena is an example of a Mitsuru that was saved by Wataru, unlike the real one who... wasn't.