Yeah... imagine someone from some random series. Then imagine some other version of them coming in, different outcomes in life, and basically being an enemy.
Like Gamma Reimu from a Touhou fanfic I'm thinking up. She's a horrid shell of a human being... and she, too, can travel through dimensions, making her more of a threat then most of the alternate world villains.
Except the Humans of the Fake Moon dimension. The ones that overpowered the youkai with technology and now rule the magical world. They keep the magic going, but they make sure that it is reigned in by Technology.
Gamma Reimu and Akira are bastards, though.