Her title in Imperishable Night is 半人半霊, which is "half-human half-phantom" on the Touhou Wiki, and 半霊 on Pixiv turns up Youmu pretty much exclusively. I can't decide whether there is a case for specifically using "half-phantom" instead of "half-ghost" or not -- I suspect some investigation into 幽霊 yuurei vs. 亡霊 bourei may clear that up, since Youmu's race is also given as 人間と幽霊のハーフ ("biracial human and yuurei") in Perfect Cherry Blossom's supplementary materials -- but certainly one of those two would be all right.
jxh2154 said: What's the original Japanese used for her?
In the game manuals she is described as 種族:人間と幽霊のハーフ and her character titles often include 「半人半霊」 or 「半分幻」 (SWR & IaMP). If you use the latter, "half-ghost" works best.
glasnost said: I suspect some investigation into 幽霊 yuurei vs. 亡霊 bourei may clear that up
Perfect Memento in Strict Sense makes a distinction between 亡霊, the spirits of deceased humans (like Yuyuko), and 幽霊, spirits that were not necessarily ever alive (like the Prismrivers- yes, they had human counterparts, but the spirits originated from Layla's mind and not their human selves.) I believe the terms 'ghost' for 亡霊 and 'phantom' for 幽霊, as are used on the Touhou wiki, convey the appropriate nuance in this case.
So yeah, I recommend "half-phantom." Or something that's not "ghost", which implies that half of Youmu was once alive and then died.
亡霊 has a specific reference to the dead, or deceased spirits, while 幽霊 doesn't. They are clearly different in meaning, and the Ghost/Phantom labels fit about as well as you are going to manage in english.
I acknowledge that there is a distinction in meaning, but I am not convinced that the words "ghost" and "phantom" appropriately capture this distinction in English. At least, I do not think we should use "half-phantom" over "half-ghost" based on that nuance.
7HS said: [...] and 幽霊, spirits that were not necessarily ever alive (like the Prismrivers [...]
To further complicate things, the Prismrivers are actually 騒霊 (literally "noisy spirits", so I don't think there's any argument that 'poltergeist' is definitely the right choice here). Despite never having been alive, the Touhou Wiki specifically states them to be closer to ghosts than phantoms.
That said, while "ghost" is probably the best choice for 亡霊 and so Youmu's non-human half should ideally be described as something else, it'd be an uphill fight - "ghost" is the first term people are likely to think of. (It doesn't help that Myon looks quite a bit like an even-more-blobby Casper .) I'd say that "half-phantom" is the better choice, but "half-ghost" isn't so wrong as to be worth the effort to correct if someone's using it, except perhaps when the distinction matters for wordplay. (For instance, post #637725 - though Youmu herself is described as 半霊 there, for which "half-spirit" is probably the best translation, so that just makes matters worse...)
Quick edit to clarify: I'm not saying that "phantom" is a notably better translation for 幽霊 than "ghost"; I can't think of any single suitable word in English that fits but doesn't have the association with dead people. I'm saying saying that if we use "ghost" for 亡霊 - which most people do - then we shouldn't also use it for a different Japanese word, since that causes confusion.
I don't think you should rely on people's understanding of the words "ghost" and "phantom" in the translation of this phenomenon in any context. But we're digressing.