Let’s start with something wholesome - tales of romances between two people where one of the people involved just happens to be nonhuman.
A Woman Marries a Dwarf
A young woman from Braderup on Sylt island had to do many hard chores every day, like most women on the Frisian islands. She often felt that she was unfortunate, and secretly envied the dwarves, who are always happy but rarely had to work. Once, a neighbor and her went to the field for work, and they came past a hill where it was frequently possible to hear the subterraneans sing and dance. “Oh”, she exclaimed, “if we could also live like the people there down below!” “Do you actually want to be with them?”, the other woman asked. “Well, why not?”, she replied. A dwarf heard this, and when the woman came past again the next morning, he proposed to her, led her into his hill, and married her. She supposedly lived there in happiness, and bore several children to the dwarf.
(From the schoolteacher Hansen on Sylt.)
 Braderup is now part of the Wenningstedt-Braderup municipality.
Translation note: The woman is described as “Mädchen” in the text, which today means “girl”/”female child” - but in older times, the term encompassed young, unmarried women. We should therefore not assume that she was underage, and thus I translated it as “woman” instead of “girl”. And, despite stereotypes, German women in previous centuries usually married in their mid-twenties, so she might not have been underage by modern standards either.
Commentary: This story initially seems like it is going to set up a moral lesson in the veins of “be careful what you wish for”, “be content with what you have”, and “nothing good comes to those who want to shirk work”, but no - the twist is that it all works out just fine for the woman, and she apparently lives happily ever after with her dwarf husband.
The Dwarf Türliwirli
A man from Oberems married Türliwirli, the daughter of a dwarf. The woman asked him one day to never call her by her name, which he promised. In June, he went off to do his share of the communal work at the alpine meadow, and when he came back late in the evening, his wife told him that she had had a tough time that day. For the next night there would be frost, and thus she had cut the unripe grain stalks and put them between fir branches. The man got upset and shouted: “You stupid Türliwirli!”, but as soon as he had said this, she was out of the door and had vanished. During the night there was frost, and the seeds of the neighbors died off.
The man had three children, whom he left at home whenever he went away for work. After he had left, the mother appeared every morning, and washed and combed them. In this manner, whenever the man returned home, the chamber was cleaned, and the children were washed and dressed properly. Then he asked who had been doing all this, since he had locked the house and hidden the key. The children exclaimed that their mother had visited and taken care of all of this. The father had a great longing to see his wife again, and he would have liked to apologize to her, if only she had shown herself. Thus, he told the children that they ought to ask their mother how she managed to get into the locked house.
When the children questioned the mother about this, she replied that she knew very well where the key was located. The distressed father now asked a friend to watch out for her, and, once the woman had stepped into the house, close the door and call for him. This was accomplished, and the father joyfully hurried into the house and asked the woman for forgiveness. After that, they still lived together happily for many years.
 Called “Alpwerk” in German. The alpine meadows were usually maintained by whole communities, and community members had to participate in their maintenance (such as building or repairing the alpine huts).
 To be specific, she put them between “Tannenreiser”. “Reiser”, or “Reisig”, are thin branches that have fallen on the forest floor, and can be picked up and taken away by anyone without this being considered theft. They are most often used as fire starters, but sometimes for brooms. The best English-language approximation I am aware of is “broom-wood”.
Commentary: Many “supernatural bride” tales feature some kind of taboo that the husband is not allowed to violate on pain of her leaving him. What makes this particular story unusual is that the couple actually manages to meet up again and reconcile - usually, the human spouse only has one chance, and if they screw it up their relationship is over forever.
The Farmer and the Forest Maiden
The farm called “Untermoser” is part of the village of Platzers near Meran. One of the farmers of Untermoser married a forest maiden. They had children, and lived in happiness. The man didn’t know the name of his wife, and she had commanded him to never ask about it. Once, the forest woman grazed in the garden at the house, and another forest maiden passed by who called her by the name “Gertraud”. The farmer had been hidden from view, heard the name, and said: “Now I know that your name is Gertraud!” Then the forest woman wept and said: “Now I must leave you forever!” Upon this, she took an iron rod, and stuck it into the field with the words: “As long as a single vein remains of this iron rod, everyone in the Untermoser farm will live well.” This has remained true until this day. Later on, Gertraud frequently came into the house and cleaned her children, but nobody perceived her.
 Called “Plazzoles” in Italian, this village is now part of the Tisens (Tesimo) municipality in South Tyrol, Italy.
Commentary: This is, sadly, the more typical ending for tales of this variety. While he disrespected her wishes - always a bad sign in a relationship - we get the impression that she doesn’t actually want to leave, but is compelled to do so by, perhaps, the laws of the forest people. The issue might not even be that her name is spoken aloud (since the other forest maiden does so without consequence), but that a human does so - and perhaps her husband in particular.