You know... I bet that coworker is just looking out for Kazukage. He's probably... a "NICE GUY".
Seriously, he probably kept hitting on Karen and got rejected so he's an asshole.
This was one of my favorite manga recently. With all the crap in the world these last few years, it has been good to read something that inspires feelings of warmth and kindness. I can't wait for the anime. Thank you for translating. Thank you.
My guess is that while they were in love at first, the two of them just weren't compatible and argued a lot to the point where it probably got to be too much to handle. It is a shame really.