Ori not only wouldn't say it, but doesn't quite get the concept. Adult Ku might say it. The sort of thing that makes an owl the size of a tree curse does not come up often.
Iselda definitely uses "fuck" as punctuation, it makes Cornifer sweat because sweetheart, is that necessary? So do God Tamer and Lemm.
Someone sent me an ask, so here are my opinions on whether a variety of Radiant Historia characters would say fuck.