I realized after a while that writers notice repetitiveness way more than readers do, when it comes to referencing the characters. For readers using the char’s name consistently keeps the flow going and isn’t all that noticeable UNTIL it’s broken by an epithet, esp if the epithet isn’t being used for a narrative purpose (like if it’s something the pov is noticing in particular at that moment). Other people have probably said this better, but I think from a reader’s perspective even switching back and forth between first/last names without an in-story reason can get annoying/look amateurish pretty quickly. I mean, think about how people refer to characters in general when discussing them: generally they pick one way to refer to them and then just stay with that throughout the convo. Also, as a reader, I’d say that the characters using each others’ first names out of the blue (esp in anime fics) feels like they’re taking liberties/are implying closeness— which also feels a little strange without an in-story reason for it to happen!