Well, since we are saying why now.
Youmu was something pretty deep to me, I have this one story on which the main character is pretty much me with the life I wanted to have, he goes through hell in life and everything, but in the end he's "happy" (In the sense "everything's peaceful but also so dull there's nothing to do but do my job day after day asdf"), when I started to draw the chracter in my head, I kinda based him off of Youmu for some reason, subconsciously, I didn't notice that until much later, but it kinda stuck.
Miko, on the other hand, is something much simpler, she reminds me of a certain game chracter I personally like (coughcoughnekusakurabacoughcough) which immediately gives me an idea for a whole new arc in my series (even though it's pretty short), which is amazing considering that arc was everything I needed for one of the characters to get his much-needed resolution.
Then,I started to mentally compare her to various other characters, a Saint like Byakuren, a Human like Marisa/Sakuya/Reimu/Sanae, a Swordswoman like Youmu, an ability that looks weak but can actually be pretty strong, like Yuuka, and an amazing backstory, like oh so many Yukari backstory theories.
Plus her theme.
Her theme.
It's one of the very few original Touhou themes that got into my iPod, the others being A Small Desire's Starry Sky (Trance + Normal, because RAVE :geddan:), Last Remote, Lullaby For a Deserted Hell and Necrofantasia (It's just there until I find an actually good arrange of this thing that isn't Liverne's or IOSYS)
That's why I love them both and why I wanna write a Youmu Vs Miko fanfiction so badly and make it canon on my series (Latter is gonna be easy, former not so much)