City of Reality/WMG

Our universe exploded; the Reality multiverse is the result
At some point in the not-particularly-distant future, our universe exploded into many much smaller universes. In at least some, humans managed to rebuild society, although varying conditions resulted in interesting results. Walk World became a global village, apparently being a miniature America; however, rather than existing in pockets, the bad parts of the world were spread out, making Walk World a Crapsack World. Magic World had a build-up of magic, and the City of Reality was clearly a social experiment that would have gone perfectly were it not for the connections between other universes and the city's reliance on them for sustainability. Of course, in other universes, entirely new sentient beings came into existence, shaped by their respective universes, hence universes like Dark World.

Cassi is Real
In what form she actually exists, and why Victor is the only one who can see her, is entirely open to speculation. But the mayor believes Victor, so why shouldn't the readers?