Because they entirely lack reference images, and because they only get about a paragraph of description in a single novel, here's the deal with the Sylphide: they're gelatinous blobs with pendulous fanged tendrils. Isaak von Kampfer summons them as one of his spooky wizard tricks. They eat souls, sort of. They also seem to eat blood, which gives them their nifty pink color. And they're invisible, except when they have to feed.

So whatcha got, basically, is a pink magic jellyfish. An INVISIBLE pink magic jellyfish.

...okay, invisible pink magic VAMPIRE jellyfish.

With big teeth.

Yeah. :D