What's a Black Rock Shooter? ...can I eat it? :<
Gather 'round and I shall tell 'ye a tale. It's actually kind of confusing where BRS came from...
Once upon a time an artist going by 'Huke' posted a character illustration of Black Rock Shooter on Pixiv (a Japanese art sharing site). At that time that's really all there was to it, like so many other illustrations on that site. As it happened Ryo from the band Supercell saw the illustration, like it, and decided to make a song about it. He used the vocaloid Hatsune Miku to do it and this was right around the time the whole vocaloid phenomena was starting to take off. Huke then joined in to provide illustrations for the accompaning music video.
The video proved to be extremely popular and has had millions of views. Then the merchandising started. Good Smile Company got involved and started making BRS figures, a short straight-to-video anime was made, and there's even a manga. And now there's this game. The actual stories are all over the place, though. There isn't actually any connection between the anime OVA and this game, other than that they both feature BRS. I believe there is actually another anime in the works that will be a TV series starting in January.
So yeah, the origins of this game are saturated with Japanese fandom.