Mrs. de Winter portrays Frank in a very sympathetic light. He's described as the quintessentially proper English gentleman and the perfect host. He's infinitely kind to Mrs. de Winter, and he's always at her side to support her in difficult Manderley moments.
According to Maxim, Rebecca pursued Frank sexually, and Frank had to call on Maxim to make her stop. According to Favell, Frank pursued Rebecca, and turned on her when she wouldn't reciprocate. In this way, Frank is a great example of how we never really know what to think about these characters because their past is so mysterious (and all we know is what Mrs. de Winter tells us). Who do you believe?
It seems like Frank knows that Maxim killed Rebecca and actively participates in the cover-up. He's even prepared to pay off Favell when he tries to blackmail Maxim with the letter he has from Rebecca. Like most characters in Rebecca, Frank has a dual nature: part good, part evil (and part that murky territory in between).