@Tamerlane @_i_is_weeaboo_ This manga's "progress" is all over the place, so not sure about the "gradient" thing, but anyway. I'd say first was Chapter 4, when he gave her his heroine drawing. She blushed and didn't want to share with her friends.
@CommieJohnTeh As someone whos binging, I can say Chapter 4 is the absolute first, if you're willing to accept receiving a picture and blushing as starting to have interest.
Also, it is NOT a progression. We go from that, to 0 progress, to her giving him a metric ton of choco in an Extra, to 0 progress again, to her yelling "Suki da" in front of the school (chocomint, probably just to tease him. Point is, she didn't care who heard), to her not being able to tell him "I was waiting for you" at the school gates, to her not being able to ask for his Line.
I mean, if you call one step forward and 70 steps backs progress, then by all means...