When creating a character who is supposed to be an honest person, there are multiple ways to showcase this trait. The use of profanity is one of them. Learn how to use profanity to make your character not only seem more believable, but more trustworthy.
I don’t know of a single person in my life who I have a close relationship with who I’ve literally never heard use profanity (even if they don’t think I’ve ever heard them!).
There’s a good reason for this. Profane language has a specific flavor in our vocabulary that simply cannot be substituted. “Darn” is not the same as “damn” just like “frustrated” is not the same as “furious.” Yes, they’re related, but which word you choose to use gives information about the context, you as an author, and your character as a person. After an argument with their wife, does your character mumble “bitch” or “witch” under their breath? When protecting their grandchild from a raging house fire, does your character tell them to get “the hell out of here” or “the heck out of here”? How your character responds in these types of situations can be an opportunity for you to show the reader what your character is like instead of overloading them with narrative and “telling” them.
That said, this doesn’t mean that you cannot possibly have an honest character who doesn’t use profanity. Nor does this mean that your villains and other dishonest characters MUST use profanity. I’m just pointing out that you have an option to strengthen the visibility of a character’s honesty in the fact that they are profane when they speak.
Profanity has a bad rep all throughout the world. My speculation is that this is partly due to the fact that curse words are a type of “naked” and aggressive language in a world where people often want things to be softer, quieter, and more pleasant. Instead of hiding portions of meaning or intensity behind euphemisms and G-rated words, someone who uses profanity doesn’t shy away from “tellin’ it like it is,” so to speak.
Often (though not always, of course), if this character is willing to be open and up front with others, there is a certain level of honesty that they have about their own lives. Cognitive dissonance may be an issue with them much less regularly than with your other characters.
When, where, with whom, how much, and exactly which curse words are all choices that are up to you to make when it comes to building your characters mannerisms. Here are some questions to ask yourself and help figure out where you want to stop the dial on the “profani-meter” of your honest character.