Mirror War: Battle Royale

Kiyo has a relatively normal life as a highschooler, until one day he sees his childhood imaginary friend Yuna attend his school and is VERY real. He learns that the universe is devided in two almost identical worlds, except some people only exist in one world, which causes disruption. To lower the causalities, the one world people, also know as "mirror men", participate in a battel royal in which max 6 people get out alive. Little did Kiyo realize, there's no choice in believing it.