By the way, what the fuck is an OC?
Original Character: Self-insert Marty Stu dreck, the hallmark of terrible fanfics.
Not necessarily a self-insert Marty Stu. OCs can be justified by the fact that Gensokyo has a population of thousands, so think of them as filling in the areas canon material left out.
IIRC, everyone in Gensokyo got sealed inside it in the 19th century, leaving behind their families in most cases (although Marisa's family lives in Gensokyo), with many beings having a lifespan of hundreds if not thousands of years. There's no need for OCs to fill any gaps really.
Although I can totally see the fun of OCs. I was being facetious, mostly. Mostly. Just refering to our little version of the recurring flu.