Voyagers are a rare and extremely powerful type of magic-user.
In most realities, the magical and mundane are in relative balance with one another. After the Anotherworld Event, many worlds were flooded with magic, and the balance was shifted. In Gaia's case, this imbalance created a tear in the fabric of reality that split off into other worlds. It was theorized, then, that an individual magic-user could become powerful enough to create one of these tears on their own. They could physically step into the space between realities, influencing other worlds through sheer force of will. They could become something like a god, strong enough to create a reality all their own. With as much magical potential as they could find, the people of Earth decided that a human could never achieve that level of power. It was deemed impossible.
It is not impossible.
When a living being generates enough magic to pierce the fabric of reality, they are instantly and permanently removed from their world. They escape through the tear in the fabric and into the Void, the space between realities. From there, their magical potential increases, and they gain the ability to change realities at will— at the cost of never being able to return to their original world.
Voyagers tend to be fickle, whimsical. Without a home reality, they spend their time in unfamiliar worlds, collecting knowledge and magic so that they can someday create a world of their own. They tend to travel in pairs or trios, staving off infinite loneliness with the comfort of their found family.
To create a new universe, however, they will have to go it alone.