Hi so I'm trying to get the "fair favor" hat from completing serafen's quest,

The quest is completed, I saved his friend, his reputation is maxed out, I've rested, waited a while, gone further into the game, loaded my saves from before completing the quest, talked to him over and over again but I still can't get the hat from Serafen

Any ideas?

Did you have this:

 

In order to discover that his and Serafen's former captain - Bastian of The Sorcerer - had been selling the slaves they rescued, you need to have learned that the Príncipi work with slavers (requires completing Symbols of Death for Furrante and promising to help him). This will reveal the true reason that Remaro left the Príncipi , giving Serafen closure.

And afterwards (after cutting Remaro loose and leaving Sayuka) did you ancounter and fight Malnaj?