Maybe I'm alone in this but I really, really hate when translators change the order of characters names to suit their own tastes. For the first part of the chapter, I thought everyone was calling the girl by her given name until later on the "boyfriend" calls her Koharu and she explodes out of embarrassment.
They have a different culture so please don't apply our own standards of "First Name/Last Name" when translating; if anything you should consider that their first name is always the family name and the last one is their given name.
Especially when their names play such a big role in storytelling, lovers and friends deciding when to use the given name, characters transported to another world that decide to introduce themselves with the given name first, etc.