So... I see on Touhou Wiki her species is labeled as "Mermaid", but I don't think she is exactly that. She is actually a
Ningyo (The youkai equivalent to the western mermaid).
You might want to say "Oh, but they are the same thing" and I will reply with a no. There are some key differences between the two creatures.
First of all, let's start with the etymology of both "Mermaid" and "Ningyo"
"Mermaid" comes from the Old English word "mere" (sea) and "maid" (girl). While "Ningyo" (人魚) is a combination of the characters 人 (human) and 魚 (fish). I'm sure nobody will disagree that "sea girl" ≠ "human fish".
Secondly, let's take a look at Wakasagihime's omake on DDC:
○1面ボス 淡水に棲む人魚
わかさぎ姫
Wakasagihime
種族:人魚
能力:水中だと力が増す程度の能力
おっとり系の淡水人魚。
普段は歌を歌ったり、石を拾ったりして暮している大人しい妖怪。
人間に敵対心は持っていない。
打ち出の小槌の魔力に冒され、強気になっていたところ霊夢達に
成敗された。
今はもう大人しい。
The underlined segment translates as
Race: Ningyo. If she was really a "mermaid", then Zun would have put a
種族:マーメイド instead. (マーメイド = Māmeido; which is the kana used by the Japanese to refer to western mermaids)
Thirdly, Wakasagihime herself doesn't have the features of a western mermaid. Mainly due to three reasons:
1) She has a pure Japanese name.
2) She dresses in oriental clothes (a kimono to be more specific).
3) She is based after a Japanese fish: The
wakasagi (which is a species of fish native to Japan). If she was really a mermaid and not a ningyo, then she would at least have been based after a western fish, right?
The point I'm trying to reach is that mermaid is NOT the same thing as ningyo. If that was the case, then you might as well call Suika an "ogre" or "demon", instead of "oni". Since this is what the character 鬼 translates to when converted to English. You can confirm by visiting
the Japanese wikipedia article of the western ogre and reading the last segment:
"日本では「鬼」と訳されることが多い。" translates roughly as: "in Japan it's often called 鬼"
Therefore, I'd suggest someone changing Wakasagihime's species to "Ningyo" on Touhou wiki.