Ok so what I’ve gathered so far is this:
1. Donna was a legal secretary at the DA’s office before Harvey knew who she was.
2. Donna has always been really good at putting her guard up around Harvey.
3. Donna knows everything.
Here’s the think part:
So Donna knew him before he knew her. My guess is she had a crush on him. She knew she had to make herself stand out when she met him, so she wouldn’t be someone he forgot easily. She had to make herself irreplaceable. I think Donna had a huge crush on him, and as such learned all kinds of weird stuff about him. And because Harvey (apparently) has a thing for red, he was smitten. Also she is super practiced in putting up that fake unaffected expression. By maintaining her illusion of dominance in the relationship, she is able to keep hidden the fact that he has the actual power. By that I mean, that he is the one with the ability to annihilate her. I just think its an interesting and believable concept that Donna came into the relationship already pining for him.
Someone should write a fanfic…