The only chapter where blondie doesn't look like the maniac is this one, yeah, sure, it all seems to pay off in the end but honestly, what was so bad about her work-aholic attitude that she needed to change? A few shitty co-workers talking behind her back
I think she was unwilling to assert herself or be herself. If the maniac was able to totally change her personality, she'd lose the woman she fell in love with, but I think that's unlikely. Anyway as many commenters said, Kouhai probably ended up going too far, which is why they aren't together at Christmas. It's not that they talked about the MC behind her back, it's that she had no respect at all, even self-respect, and they were willing to kill her with overwork. As long as she was willing to put up with it, they'd load all their work on her and goof off. so she's not even helping them be more productive.
The kouhai is a CLASSIC "Manic Pixie Dream Girl." If the MC were a male, most of the criticisms of the kouhai wouldn't exist. By nature, MPDGs have a certain amount of unreality to them. MPDGs barge into your life and teach you how to LIVE with their zany outlook and hyper energy, you workaholic hermit.