Omg because she's Japanese having his last name first sounds like it's her surname. But because she has a western first name, having it after results in the same thing. So she has to use his first name aaaaah
Her given name isn't actually western - translation notes near the start of the series pointed out it is an actual native Japanese name that just sounds like a western name.
Chapter 1, page 6, for reference, all the way in the lower right corner.