@Kaarme ,
@Attac_Mage : It is not as weird as one would think. In "western" cultures it is pretty common to call others by their first names as soon as you get a bit closer, e.g. friends or maybe just anyone you get to know out of work. In Japan, you could be friends with someone and don't even know their first name because you never hear it spoken. Except for if you're both girls, then you would probably use first names. But you can feel closer to someone by leaving out the suffix for example, or making up some nickname. How you call your friends or significant others depends heavily on where you got to know them - e.g. when you got to know each other in a work context, you would call them by the surname or when your friends all call the person by a nickname you might do that too (sometimes, when that is really not appropriate to use but you're so used to hearing it, you stumble - like what happened in the chapter). When people get into a relationship, some might then decide on calling each other by first name or some new nickname and some may stick to the names they have so many fond memories with.
An analogon would be maybe if your friends call someone by a nickname only, then you start dating that person and its just more fun to call them like you're used to.