Ah, yes, OC's, a microcosm of your own character development. You draw from your own experiences about the world to create them and call for them to help guide you in the eternal march of life towards death.
My OC... well, there's an entire history for her. While I never designed her to be a Gensokyan, the entire reason I created her was as a self-insert that could fit everywhere.
Originally called Nyoko Hakui, she first went through multiple story arcs as I was going through them in FNAF and later Undertale. I don't remember every single franchise she went through, but eventually, likely due to a desire to stave off boredom, she got killed. There was another OC ready to replace her, of course. This new person, named Shinso Hakui, came into being shortly after I discovered Touhou, I think. Yet my imagination acts in mysterious ways where I have to work to actually catch a story, so she might've existed before then.
Why all that history? Well, I have had to attend enough funerals for a lifetime, and my parents still live... I suppose Shinso here was my brain trying to grasp the concept of having my parents funeral in my lifetime.
Shinso, while filling the role of Nyoko, still has a different personality. In a way, if I ever were to cross-reference the decisions Shinso made with the decisions Nyoko would have made, I'd come with two different universes.
Shinso, while maybe not made for Gensokyo again, does enjoy living in Gensokyo and if I were to ever retire the character, she would choose the place. As for her actual place she comes from... well, I call it Yru.
Yru is a future where the Earth has gone to hell and hasn't come back yet. Humanity got destroyed by aliens and experimented upon by them turning some into creatures similar to catgirls (with two tails and they call themselves Nekomata... I'm not hiding my inspiration there.) or people with wings for arms. Those were then sent to a planet named Yru that the aliens commit suicide on. The same substance that causes the aliens to commit suicide also summons lesser beings of a faith, (It does not care which.) and allows some people to perform feats of magic. It is in this Universe that Shinso was born in, as a (Shinto) Nekomata, she exists purely for vengeance. Her adoration of humans prevents her from being vengeful towards her, so she directs her anger towards the aliens who put the humans in this poor position. I'll also mention that those aliens caused a particular part of the planet to always be devoid of a sun.
Yet she is also powerful enough to be considered a Kasha, which makes her an interesting character in her own right.
She appears as a Nekomata in her everyday form, wearing the usual black full-body suit of the Nekomata of the Eternal Night. Though she has been trying out more casual clothes as of recent. She has full-on bloodred eyes, though not bloodshot ones. Quite tall for our standards, standing at just over two meters. But average for that universe, and considering that Shinso needs to fit in, you can infer why she chose that tall. I explain this with the real world example of breeding to make people giant, which worked in Prussia and Darwin even gave that as an example for his theory of evolution. She has pure black hair and keeps semi-long so it doesn't get in the way of her view when battling.
Thanks for reading this far. I encourage you to be inquisitive about things you don't understand or want more information about; I'll be happy to answer you... Though now I have my reservations about the legality of such things. I can hardly remain on topic for half the post, now can I?