As a cult series of 20th-century children’s books, the Moomins have sold up to 30m copies worldwide. Now, extensive humorous notes that their Finnish creator, Tove Jansson, wrote on each of her lovable trolls with hippopotamus snouts are to be published for the first time, 25 years after her death.

Eighty-nine handwritten pages that cast new light on the “small, friendly and adventurous” creatures with fur “like velvet”, have been rediscovered among hundreds of thousands of items in her sprawling archive.

James Zambra, her great-nephew and a director of Moomin Characters, which manages her legacy, said: “This was actually in one of her notebooks. It’s fantastic. Getting Tove’s own thoughts on the personality traits of the characters is fascinating.”

The Moomin books, which Jansson wrote and illustrated between 1945 and 1977, are about a family and their friends who live in harmony with nature in the magical Moominvalley. Their adventures inspired adaptations for screen and stage as well as two theme parks.

Jansson’s unpublished notes are thought to have been created to guide directors because the descriptions extend to the voices of each character. Zambra said: “It’s fascinating to get her own take on them. There are lots of very interesting things that I didn’t know before.”

The characters include gentle Moomin, his ever-resourceful Moominmamma and adventurous Moominpappa, and minuscule Little My, adopted by them because “no one else dared”.

In her unpublished notes, Jansson wrote of Little My: “She is both aggressive and cheerful … She has no sense of respect but can show a kind of negligent friendliness when it suits her. If she bites you, it will mean nothing in particular, being simply the sort of thing she does.”

Jansson imagined the character’s voice as “happily hoarse but never shrill … with a sudden almost brutal laugh”.

Describing Moomintroll, she wrote: “He has a tendency to romanticise and be compassionate, traits which often cause him confusion … He feels sorry for everything that is small, frightened and broken … He is covered by something like white velvet and is inordinately proud of his tail … Moomintroll has an eager boy’s voice that sometimes tips into insecurity.”

The notes will feature in a special edition of Jansson’s first story, The Moomins and the Great Flood, by the independent publisher Sort Of Books to mark the books’ 80th anniversary. It enables readers to build their own 3D Moominhouse based on Jansson’s designs from 1957, which are included.

Natania Jansz, the publisher and a Jansson expert, said the notes revealed “the psychological depth with which she imagined these characters”. “The descriptions are extraordinary,” she said.

She added that in The Moomins and the Great Flood, the characters have become refugees after a natural disaster: “It was published in 1945. She wrote it at the end of the war when, after a period of despair, everyone was in a state of flux. Millions were displaced. So this reminds us of that period and echoes our own.”

In the book’s foreword, Frank Cottrell-Boyce, the Waterstones Children’s Laureate, writes: “While Jansson was writing this book, the roads of Europe were crowded with families, refugees trying to rebuild their lives – and find lost loved ones – at the end of a global war. The same is true today of course, in Ukraine, the Middle East, and Africa … This is the important thing about Tove Jansson. Yes, her stories are full of wonder and invention but they are also about the world around her … The stories ask us to arm ourselves not with magical swords or rings of power, but with kindness.”

Jansson supported numerous charities, and on Thursday there will be an announcement about new donations. In an agreement between Waterstones and Sort Of Books, the sales profits from the first 4,000 copies of the new book will go to the British Red Cross, while Moomin Characters will make a six-figure donation to the International Federation of Red Cross and Red Crescent Societies.

Moomin Characters is also partnering with Refugee Week, an arts festival, for a 2025 nationwide project in which four public artworks inspired by the Moomins are being created, co-commissioned and co-produced by Counterpoints Arts, an organisation devoted to the “arts, migration and cultural change”.

The commissioned artists are Dana Olărescu, Basel Zaraa, Henna Asikainen, and The Woodland Tribe with Nabil Amini, each inspired by the Moominhouse and values in the Moomin stories, “in connection with contemporary experiences relating to displacement”.

• The Moomins and the Great Flood by Tove Jansson (Sort of Books, £12.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.