Did they ever list a date for when these past lives were? Because based on that military uniform it seems like Ryuu probably died around 1920 or so. It was definitely sometime before 1945 since Japan hasn't done the draft since then, plus it was a time before they had Christmas in Japan.
That hit so much hard, first expression was a fundamental to their relationship, but he never had a chance to fully express his feelings through her tsundare personality towards him...