I've been thinking about the newest update and I realized that it makes perfect sense why Frankenstein of all people would be the first to both see the transformation and know that Jekyll and Hyde are the same person. She's the first person to ever see the cracks in Jekyll's facade and is sort of what triggered them in the first place (as in she's the first person to ever see green-eyed Jekyll). Also, as far as we know, she is the only person that is completely aware of Jekyll's scientific research. Sure, other characters know that he's into science and that he's an alchemist, but if memory serves, Frankenstein is the only one that knows that Jekyll was specifically working on how to separate "good" and "evil" (and then dunked on him for not having concrete proof of this and for studying something so arbitrary but that's beside the point). I can imagine that after seeing that whole fiasco she started putting two and two together as to what the hell was going on. I mean, it's not like she hasn't seen that same shade of acid green eyes on someone else's face, nor heard even a passing description of what this "Mr. Hyde" looked like