Alternate

Near endless realities.
One that is above all, occupied solely by a race called the Pure.
Their job? To ensure humanity knows nothing about these realities, so they don't end up killing themselves.
It's not a simple task. While natural portals are virtually impossible to come by, there is another force at work. The beings from the plane of existence between realities-- Voidwalkers. These will do anything to escape and expand into the real worlds, and create space distortions anywhere they step outside of the Void, creating artificial portals.
The Pure aren't stupid. Centuries of experience has created an elaborate system of dealing with Voidwalkers, and detecting disruptions in the balance of Realities. The Pure also exist to return any lost humans to their home Reality, if two of the same person, "Alternates," are detected in the same Reality. Besides belonging to the King Reality, what makes the Pure so special is the lack of having any sort of Alternate, unlike humans.
Enter Kalin, The youngest Dimensional Slayer to be seen on the force, "Voidhunters."
Besides from standing out with his odd, bright purple hair, he is often looked down upon, for having completely skipped the experience required in the police division, but with the help of his squad, he feels like he can get through being slightly different, and Kalin could become a regular, Pure Voidhunter.
That is, until the one, fateful day, where Kalin encounters his Alternate.
And he isn't human.