Tove Jansson (1914-2001) is a Swedish-speaking Finnish author, most notably known for her Moomin character books, which she published starting in 1945. Though she is often remembered as a children literature author, she has published extensively also for adult audiences.
The popularity of the hippopotamus-resembling Moomin characters was undoubtedly due to the storyline, which was doubled by Tove Jansson‘s own illustrations. Since 1992, they have become a constant presence on stamps, from their native Finland and off to the world.
Åland Isl. issued this stamp featuring the art of Tove Jansson in the year 2007.
I would be totally unaware of the popularity of the Moomin characters in Japan if it were not for their already two-time stamp issue representing the characters.