Known as The Fillyjonk in some English translations, the first we hear of is the late theatre director in Moominsummer Madness (the Rat is his widow) and the young Fillyjonk who joins the Snork Maiden in the late night jaunt to the wishing well.

Later we meet the wonderful psychological study of the Fillyjonk Who Believed In Disasters in Tales from Moominvalley. Not a single moment of fantasy or joy, only duties and discipline; she is an extremely methodical person tied down with principles and has a deep rooted belief in prestige and tradition. Nevertheless, after a catastrophe, The Fillyjonk can be freed from the trammels of social expectation and can discover the joys of freedom, irrationality and self-expression. Deep inside she has had a wish to live freely as the Moomin family does, without any worries. Fillyjonks resemble humanoid rodents; they are tall and thin, with long muzzles and raccoon-like rings around their eyes.

(source: wikipedia)