The ballad seems to ba part of a corpus of stories/songs about relationships between humans and water spirits. The Outlandish Knight may be part of the same group - it ends by providing an alternate ending where the German ballad starts, with the woman throwing the alien being into the river and making her escape.

At the other end of the story, The Wife of Usher's Well has three children coming back from the water to revisit their mother. It's been refashioned into a conventional ghost story, but you can see how a different spin on the events could have made it a sequel to Lilofee's story.

And as Mo says, the Irish/Orkney version reverses roles.