I run into this problem semi-regularly when I join new fandoms and then have to figure out what names to use in tags when bookmarking fic to my pinboard. When I'm still new to a fandom and haven't yet settled into whatever's most comfortable to call the characters it's hard to know what version of the name to call them! Is it the name they call themself in their internal narration, is it the name most other characters call them, is it the name that third-person prose in canon calls them, is it a common nickname, etc. Depending on the fandom, it will end up being very different answers! I sometimes have to go back and change all my tags after I've been reading in the fandom for a little while. And sometimes I disagree with my past self's choices but then never quite get around to fixing the tags so I have some weird tags in places.

Anyway possibly my most idiosyncratic choice of what name to call a character is that I always call Jean Prouvaire by Jean Prouvaire or Prouvaire, as VHuggs does, rather than Jehan the way Amis fandom does. Jehan is canonically established as a name he chooses to go by among friends so that's extremely valid of people to call him, but to me it feels weird to be on a more intimate name-basis with him than his author is. So Prouvaire it is. (Also then it matches the rest of the Amis who don't get any first name.)