Warning: Queen Wasp and Catalyst spoilers!
Have any of you ever noticed how Nathalie has managed to work for Gabriel Agreste for years and years, regardless of how cold and impersonal he is? In the later episodes of Season 2 we found out that Nathalie is aware of Gabriel Agreste's Hawkmoth persona, which in my opinion is quite interesting, seeing as he doesn't trust his own son with the secret.
Nathalie, it appears, has been a faithful employee of Gabriel Agreste for years and years. Her cold personality also matches his. In the episode Queen Wasp, however, we got to see a side of Nathalie that had not been revealed to us before. She is aware that Gabriel has secretly hidden Emilie in one of his many hideouts. Not only that, but it appears as though she was helping him all along with his Hawkmoth plan. That being said, it seems as if Nathalie also has some sort of interest in Gabriel (as evidenced by Queen Wasp.)
At the end of Queen Wasp, Gabriel/Hawkmoth, seriously contemplates giving up his whole charade due to his latest failure and it appears that, for once, Nathalie actually seemed slightly happy. Let's get some facts straight here, Gabriel Agreste is a 'widow' of some sort, with a young son he has no idea how to communicate with. Not only that, but he has somewhat questionable social skills and a great taste in high fashion. Nathalie is your classic overworked personal assistant, but there's more depth to her character. There's a story behind Nathalie that we haven't been able to discover yet.
Once Gabriel decides to keep on fighting to bring Emilie back, you can see the sliver of hope gone from Nathalie's eyes. Here's my theory, there's no way anyone in their right mind would stay working for Gabriel Agreste for so long unless they have a really good reason. I believe that Nathalie's reason is that she cares, but since Gabriel is still not over whatever it is that happened to his wife, Nathalie has decided to shove her feelings under a rock and keep being a loyal companion to the otherwise very lonely Gabriel.
In Catalyst we see something else, Nathalie is aware that there's no stopping Gabriel now. She's also aware and knows quite well of his master plan, which has only been revealed to us viewers in part. She's willing to become akumatized on purpose just to help him. Why? Once again, because she cares. My question is, why would she be akumatized? Akumas only work when a negative emotion is present... A great example of a negative emotion is the feeling of rejection. No matter how deep, rejection is still there, and it could be a deciding factor in Nathalie's akumatization. Then again, as Chatte Bleu asked, is it rejection, or unrequited love?