FOR GERMAN FASHION DESIGNER Jil Sander, everything is a question of integrity. When she quit her company the first time, in 2000, it was over the quality of production: She would not accept what she saw as proposed cuts by the brand's then-owner, Prada CEO Patrizio Bertelli, for the sake of profits. When she quit a second time, in 2004, she says, it was over the direction that Bertelli wanted to take the brand, turning it from a fashion house into an accessories-driven business.

Now that she has returned yet again, having...