Udo of Aachen
According to the article, Udo was a mystic and poet whose work was set to music by Carl Orff with the haunting O Fortuna in Carmina Burana — actually the work of itinerant goliards, found in the German Benedictine monastery of Benediktbeuern Abbey.
Udo was also an illustrator and theologian, supposedly he discovered the Mandelbrot set, some 700 years before Benoît Mandelbrot. His works were rediscovered by the also-fictional Bob Schipke, a Harvard mathematician, who saw a picture of the Mandelbrot set in an illumination for a 13th century carol.
The monk's supposed finding was lent an air of credibility because often medieval monks did discover scientific and mathematical theories, only to have them hidden or shelved due to persecution or simply ignored because publication prior to the invention of the printing press was difficult at best. Mr. Girvan adds to this suggestion by associating Udo with several other more legitimate discoveries where an author was considered ahead of his time in terms of a scientific theory of some sort that is now established as a mainstream theory but was considered fringe science at the time.
The other aspect of the deception was that it was very common for pre-20th century mathematicians to spend incredible amounts of time on hand calculations such as a logarithm table or trigonometric functions. Calculating all of the points for a Mandelbrot set is a comparable activity that would seem tedious today but would be routine for people of the time. That a 13th century monk would spend his time doing some apparently meaningless calculation increased confidence in the accuracy of the story.