It's definetely japanese. What I remember is a little girl around 13 yo getting a partner marriage with adult male. I remember the setting probably not in modern era.
The adult man doesnt immediately build a romance relationship since she is still a child after all but more like a brother/father figure for her. (I think he is waiting for her to grow up)
My memory is fuzzy cause I don't read that far too. If I recall the girl wears kimono at the begining chapter.
Edit : Probably (not sure) the man also wear a uniform too at the begining chapter (not sure if its navy/military/civil servant)
The adult man doesnt immediately build a romance relationship since she is still a child after all but more like a brother/father figure for her. (I think he is waiting for her to grow up)
My memory is fuzzy cause I don't read that far too. If I recall the girl wears kimono at the begining chapter.
Edit : Probably (not sure) the man also wear a uniform too at the begining chapter (not sure if its navy/military/civil servant)