Board Thread:General Discussion/@comment-4435241-20191001200320/@comment-9623384-20191025202953

Like SyckInsanity said, it's about their relationships/friendships. For Tenma, most of GO's Raimon members also called him Matsukaze until much later. Haizaki hates when Kidou called him Ryouhei. They don't called each other by their names unless they are close enough.

It's also about the formality, surname is more formal, like the commentator always call the players by their surnames. And there's also an honorific thing, the younger one should talk to the older one with polite words. Actually both Tsurugi and Haizaki can be seen as being rude, as they called every older members without -san. But Tsurugi becomes more polite after he changed his attitude.

It's also depending on themselves, for example Toramaru prefers to be called by his name, but this also goes back to the honorific, because he knew he's the youngest.