Does Nue have a true form? In some of the extra stages of UFO, it implies that her true form is her humanoid form. For example, Nue says things like "I was just thinking...I'd better do something before you hurt my true form" and "But now that you've seen me, I'll have to bury you so I can go back to being an unknown." However, in SoPM, Nue is described as " a youkai whose real form cannot be established."
I've always found this funny because it doesn't make any sense for Nue to just decide to show you her true form for no reason and then complain that she's shown you her true form. On the other hand, it would make sense for her to try and trick you into thinking it's her true form. She might as well have said that for any form she could have taken, and you would have to believe her just the same.
In my view, her "true" form just means it's the form she prefers to take to represent herself. Especially considering youkai phase into existence, a true shapeshifter by nature shouldn't have any one true form. However, for the purpose of defining a character, yes, this would be her true form. Her only other form is the nueball anyway, as far as we know.
If I offend or make it seem silly, I apologize, as I am still quite new to Touhou and I learn a great many things about it everyday (some of which seems to be fandom). It only makes it harder when I try to sort out the facts in my head and often get myself confused on some of the issues. I usually don't speak with people who are so knowledgeable on the facts.
A pointer, the official books are a very important resource on Gensokyo. They very often contain a great deal more of
detailed information about characters, places and the world, than the games do.
Very rarely do characters actually return in later games; generally the only times they do is if they're a player character, it's a camera game, or it's a versus game. However, many characters do reappear in written works, especially in the various manga. The written works, unlike the games, have a chance to show you the life in Gensokyo when the protagonists aren't going out to solve some incident. This is naturally where you see the various previously-established characters and places reappearing.