The real explanation this manga needs to give is why the girl joined the evil organisation in the first place.
She obviously does not fit that role in any way.
So how did she join that organisation and even more so how did she obtain a leadership position there?
If I had to guess, she was bullied in school, throw in some mumbojumbo about sensing the vast amount of negativity in her, a natural affinity for violence (just look at the combo on her lover), and whammo: Scantily-clad Evil enforcer.
What i seen a same manga like this... Where there were two grouos in a school or something... That manga was boring as hell and i can already smell the content that this manga is gonna serve in the platter.
Reminds me of a hentai with pretty much the same setup
Start with the two already secretly dating.
But after getting 'interrupted' so many times, they were about to break up
One last goodbye sex.
Then as she was sneaking around his house afterward, she found a note basically detailing his plan for the future to marry her and have kids.
Right in the kokoro.