The thing is, most characters in Prince of Tennis call one another by their last names. So far as I know, in fact, Silver Pair are the only ones who shift in their name-calling as they grow closer over the course of the series. This is a little bit weird to me in some instances (for example; Oishi calls Eiji "Eiji." First name, no honourific. So it seems like, weird beyond words to me that Eiji calls Oishi "Oishi" and not "Shuuichirou.") Sure, it makes sense a lot of the time, but when it comes to writing pairings, well... what is one to do?
I've seen just about everything done before in fics that I've read, from both being called by last name in narration to both being called by first name in narration, regardless of what the characters think of and call one another. I've seen character switch what names they use over the course of a relationship in a fic, and characters having, in a fic, been dating for years and still calling one another by last name. I don't know what to go with... It's just an interesting debate. I've been really conservative in the past... so I'm making a resolution to try to break out of the box. XD;;;; Yosh!