@Ikari: I am assuming you have no idea what youkai are. Your replies don't make any sense.
A youkai, by its nature, is a description of a phenomenon or thing. Rumia is a darkness youkai, because she has something to do with the darkness. Yuuka is a flower youkai, because she has something to do with flowers, as already mentioned in her title. Yukari is a boundary youkai, because she has something to do with boundaries.
Why are you comparing humans to youkai? They aren't the same. As I said and I will repeat this, a character's title is a description of the character.
Reimu being able to fly doesn't mean anything, she is not a youkai. Sakuya and Marisa are bad examples, they are not youkai.
Quite the contrary, it does make her a puppeteer youkai or as Drakums says, dollmaster youkai.
I repeat, this is not a theory. This is what youkai are.
Well geez, sorry for not being able to understand your point either. I mean, you just had proof from Drakums that Youkai shouldn't be classified in types, and you keep on insisting they can, should and are. I'm going to assume you have no idea what the term ''youkai'' is either. Looks like we're in the same boat, let's hope Murasa won't sink it!
And yeah, throwing humans in there weren't the best example. I'll give you that.
And... You missed my point about Alice, but I don't really care about that because the magician race in Touhou is pretty much the most ambiguous thing ever. It's a race, it's a job and it's a science. No point in arguing on that.
Yuuka's titles:
Oriental Demon (TH04) (*cough*Youkai*cough*)
Youkai (TH05)
Beauty of Everlasting Darkness (KOG)
Flower Master of the Four Seasons (TH09, PMiSS)
I see one reference to flowers and it's borderline a reference to how seasons don't affect Yuuka's love for flowers.
Yukari's titles:
Mastermind Behind the Spiriting Away (TH07)
Border of Phantasm (TH7.5, TH10.5, TH12.3)
Youkai of Boundaries (TH08, PMiSS, TH11)
Youkai that lurks in the Boundary (TH08 character intros)
Uncanny Smile (TH9.5)
Phantasmal Wedding of the Kitsune / Phantasmal Sunshower* (TH10.5)
Again, she's called a youkai of boundaries ONCE among her 6 titles. If you're going to use titles as proofs, then I can call her the embodiment of the border of phantasm.
Anyway, I'm personally done arguing about that because honestly, it could go on forever. I'll let Drakums take over, he pretty much said in a fantastic way what I've been trying to say but have been unable to due to my lack of vocabulary and ability to expand on my point of views.