Mrs Fillyjonk (Swedish: Fillifjonkan) is the opposite of the Moomin family. 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. But in a catastrophe, Fillyjonk may behave totally irrationally. Deep inside she probably has a wish to live like the Moomin family, without any worries. She is most often accompanied by her three children.

She originated in the moomin comics as the Moomin family's stern neighbour, but in the books there seems to be multiple fillyjonks. One appears in Moominsummer Madness who invites her family to dinner every midsummer despite the fact that no one accepts her invitation. She then realises (with the help of Moomintroll and Snorkmaiden) that she doesn't need to invite them since she and her relatives hate each other. After that she becomes a much happier person and lights a midsummer fire to celebrate. Another fillyjonk worries for natural disasters and is an acquaintance of Gaffsie in the short story The Fillyjonk who believed in disasters, and in the final novel Moominvalley in November an unappy fillyjonk travells to the Moominhouse while the Moomin family is away at a lighthouse,