Natsuki! Dang, it has been since they visited the high school in their first year of college... that was at least 4 years ago.. Geez. Also, since she was 19 at the start of the series... that means Natsuki is... around 27 now?! WOW time is setting in.
Also, I'm glad Kasumi commented about the train question. Imagine being together for 7 1/2 years before considering becoming official. They don't need to, but it is an interesting thought.