Arnold is the one who transformed Leni from an obscure actor into a true film director. Fanck was a pioneer in the film industry himself, and he taught Leni the ropes. Leni played in multiple films by Fanck. The Holy Mountain (1926) was the first one. Later, in 1929, Riefenstahl featured in The White Hell of Pitz Palu, where Riefenstahl started making a name for herself.
Fanck is the go-to mentor for Leni. He encourages her to do what has not been done before, something Leni is forever grateful for. Because Leni was not in a steady relationship during these times, Fanck was the one person she could trust. He was not involved with the Nazis at all. He is even known to have gotten in trouble with Jospeh Goebbels for refusing to work on a propaganda film. This put him in financial difficulties, meaning he had to accept a contract from the Japanese, also against his will.
Fanck is stuck between a rock and a hard place. He wants to create films, but the Nazi regime forces him to lend his talent to benefit the Reich. The one thing he can take pride in, is his mentorship for Leni. Leni has built up enough credits with the Nazis to create her own work. Fanck is conflicted by this, but decides to support her and make the best possible content that still satisfies the Nazis, but also brings a more objective and less drilling version of propaganda to citizens.