Thread:Oppaichan/@comment-5975474-20150604013903/@comment-7948498-20150604111912

Japanese speech is either formal and polite, or informal and casual. They have to always be formal and respectful to people older or higher in rank, and to strangers and acquaintances at first. And being informal and casual is permitted between close friends, or else it's simply rude.

Japanese people do generally address each other by their family names, and add honorific for formality and politeness. And using first names implies close relationship and requires permission from each other. Sometimes they would even ask each other for permission.

But the reasons for using first names can vary sometimes. Sometimes first names are a good way to differentiate between siblings or people with the same family name, but still require permission. And children significantly younger don't require being addressed formally either. But oftentimes, some people of the same age are just friendly, open to everybody, and think little of it. For example, if classmates you barely know tell you off the bat to use their first names, it's basically their way of saying "let's be friends".

In Lala's case, I think I remember that she introduced herself as just Lala, and she immediately called him Rito. Run also started calling him Rito-kun. Nana calls him Rito because she's just immature and rude. And Momo calls him Rito-san, which is close but still polite and formal, the way a wife would call her husband whom she considers higher than herself. And so for Rito, calling them by their first names just came naturally. They pretty much gave him permission by default.

And Haruna, well, both she and Rito are just too chicken to get closer to each other in any way. So it's family names for them, and will be for a long time.