Parallel World

Doppel is the world parallel to Earth. There's a counterpart in Doppel for every person on Earth, but not everyone in Doppel has a counterpart on Earth. The link between a person and their counterpart works almost like a subconcious. It's extremely common that the counterpart is quite different to their original, especially in personality.

The closer you are to your other self, the easier you'll be able to cross the barrier into Doppel. It's the job of the Queen (and by extension, the Prismas) to keep the worlds ' populations as close to equal as they can, and to keep people out of worlds they shouldn't be in.

The Barrier

The barrier, called the Echo, seperates Earth and Doppel from colliding. The Prismas cross it everytime they need to go to Doppel to continue the mission they got from the Queen. They have to be careful to not get hit too hard or they can be hit through Echo and back to Earth.

Monsters

The monsters in Doppel are called Umbrants. They've always existed for as long as humans have existed, but they hadn't had a leader of sorts until fairly recently.