Vampires have a weakness to running water
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Artist's commentary
Ozeu-sama jumps into Misty Lake
From here
There is a common joke that Sakuya calls her 「おぜうさま」 ("ozeu-sama") instead, which has made it a very common nickname for her. The old pronunciation of 「お嬢様」 is "ojiyau-sama" (おぢやうさま) but in the Edo period, "jou" was colloquially written as "zeu," like "dozeu" etc.