Mitologia finlandese/folklore: dove trovare informazioni più chiare e arte di Näkki?

Näkki doesn’t have any clear and agreed on characteristics, or “canon” form using fiction terms. Näkki is a boogeyman used to scare children to be careful near water so they don’t drown, therefore only vague descriptions like the post you linked exist. To put it simply there is no “clearer information and art” because näkki doesn’t have a clear definition and is up to everyones imagination, as long as it scares kids.

Finnish Näkki is very similar to Scandinavian Näcken, except the Scandinavian one tends to be a male and often uses a violin to lure people to the water, while the Finnish version is usually female and doesn’t use an instrument as often. You won’t find any clear info about any of ancient finnish mythologies, because all we have left of our old folklore are random stories, songs and poems that were only written down in the last century or two. The characteristics and definitions of creatures vary a lot depending on the area and the story tellers.

Näkki/Näcken is often seen a shapeshifter, and may look like a floating log or similar natural feature. She may look beautiful from the front, but if you catch a glimpse of her back, you can see her true form (which looks like she’s part of nature, like bark). Like most nordic creatures, you can break Näkki’s charm by saying her name out loud.

I would not call her a demon–more like a spirit of nature.