That's kinda the issue, unfortunately.
Great development for the MC throughout, but the manga suffered heavily in the latter half due to zero development for the FMC.
Seriously, Yamada was a bit of a blackhole for the last 3 volumes, lol
I really disliked Yamada throughout most of this series.
Volume 12 - 14, specifically, were just terrible because of her.
That should have been the perfect opportunity to grow her character, but Norio just had her cry and pout throughout THREE whole volumes.
Volumes 1 - 11 were gold though...
Crying like a child, and completely unprompted at that.
I applaud for Norio for being THAT consistent with Yamada's non-existent character arc throughout this series.