Nissi are wise, good-looking, somewhat reminiscent of light elves and unlike common ghosts. They are benevolent, but playful and mischievous. They are short in stature, with particularly short legs, as is usual with dwarfs.
Seafarers generally believe in the existence of the nissi and tell tales of them. Icelandic seamen usually describe them as being the size of fully-grown men, which is understandable as they tend not to distinguish between them and ship ghosts. Some describe these Icelandic nissi as being protectors of vessels and men, as is most often the case, despite their occasional mischievousness. Their protectiveness can also be construed as concern for their own wellbeing. They fear risking their life at sea and abandon doomed ships, as they can foretell their fate, regardless of whether the nissi are kin to fairies, elves or ghosts, they are usually benevolent and joyful and friends to seafarers. they guide them in their fishing, warn them of impending dangers and predict changes in the weather. Their merriment and laughter is a sign of an impending storm, its force in proportion to the intensity of their laughter. If they appear solemn and mournful, they predict misfortune or death on board. Their abandoning a ship, however, is a sure sign of the ship being doomed to destruction.