I keep watching the show just for an explanation of those flashbacks where Hana seems to have a calm and mature personality
I've been caught up with the manga for a while before the adaptation was announced so I don't remember the early chapters all that well, but iirc that element isn't in it, so it seems like it's an original thing they're doing for the anime. Iirc, the manga just states that they went to the same high school but doesn't specify anything about their past (the scene of them meeting up on the way to school during her first year was anime original as well I believe)