The Hattifatteners are tall, thin, ghost-like creatures, resembling long white socks or thin fungi. They have round neckless heads with two round eyes. Below their heads on either side are five quivering finger-like appendages, which can also function together as hands. They are silent and serious, having neither the ability to talk nor to hear, but in contrast, their sense of feeling is extremely accurate, and they can sense even the most minor tremblings of the ground. They communicate seemingly by telepathy, and their eyes change colour depending on ambient weather conditions. Despite having unmistakably faunal characteristics, Hattifatteners grow from seeds much like plants. These seeds remain dormant for most of the year, only germinating during midsummer's eve. When the seeds germinate, they quickly grow into mature Hattifatteners.
Hattifatteners travel the sea in small boats, meeting every year on a lonely island. Their main interest is in the weather and they collectively own a barometer. They seem to have an interest in lightning storms, and such conditions electrify them making them give out a pale glow, and making them dangerous to touch. It is possible lightning may be a source of nourishment to them (therefore absorbing it). The Hattifatteners wish to reach the horizon so they forever sail towards it in hope of reaching it.
The Hattifatteners are very serious and zealous about their barometer. In Finn Family Moomintroll, a Hemulen steals their barometer, and this causes them to relentlessly pursue him until they regain it.
In the first comic story the Hattifatteners appear, although they are not mentioned by that name, and Moomin says that they are his "poor relations".
Some think the Hattifatteners are wicked, but this is due to their strangeness. In the story The Secret of the Hattifatteners that appears in Tales from Moominvalley Moominpappa tries to understand the mysterious beings. Unusually for what is ostensibly a children's book the story deals with what appears to be Moominpappa's mid-life crisis. His quest to seek out and understand them is his search for a deeper meaning in life. Moominpappa follows three Hattifatteners to their desolate island and during this trip he slowly comes to resemble them in spirit. Arriving on the island he is eventually saluted by all of them as they bow before him, and he bows back. Witnessing their obsession with the lightning storms he concludes that their secret is that they cannot experience emotions unless confronted by lightning, which is why they desire it so much. Further, he speculates that by gathering together, they can attract lightning.
- The Hattifatteners made their debut in The Moomins and the Great Flood, albeit with a noticeably different design, having two short legs instead of a single skirt-like "foot".