While studying Electrical Engineering and then training at the Geneva Bible Institute, Jefko (Jean-Frédéric Kohler) also studied drawing and illustration.

Being very keen on communicating through comic strips, he published several pages in various Christian magazines for teenagers. After seeing this work, Claude Gaasch, a scriptwriter and director of the comic Tournesol, published by LLB Editions, asked Jefko to work with them.

As a result, Jefko illustrated two Aventure de Marc Alban comic albums, Poursuite Dans La Ville (translated as "Chase Through Town" or "Pursuit in the City," 1979) and Menace sur les Maurières ("Threat Over Les Maurières," 1983), both published by LLB, with text by J.M. Bloch and Claude Gaasch. A short message from Jefko at the end of each book says that Jesus Christ transformed his life and invites the reader to know Him, too. These Christian comic albums (or "BD" as they are known in Europe) were quite popular in many French-speaking countries, including those in Africa.

In 1984 Jefko took charge of Tournesol, working as both the editor and on the graphic design and lay out of comics in the publication.

Jefko retired from Tournesol in December 2009. He resides in Switzerland.

(Tournesol is the second longest-running Christian comic magazine in the world and is still published by LLB in Valence, France.)