I have an idea for this MG. Alright, lots of ideas. Okay, so, your confusion over whether it should be a potion or curse that triggers the switch from Jekyll to Hyde, or vice-versa, go with a curse. If the dual personality thing is the basic main appeal, don't make it so they'd have to spend time mixing chemicals to bring out the other personality. It should be done at will, although it should happen randomly sometimes. In fact, I think that there should be potions made to delay the transformation. If a Jekyll is about to suddenly go Hyde while at Thanksgiving dinner, she could just take the potion and delay it another day or so. In fact, in the actual book The Strange case of Dr. Jekyll and Mr Hyde Jekyll starts turning into Hyde involuntarily, and takes the potion to change back. And Jekylls should be...what's the word...hesitant and a bit scared to let others see the transformation. So, if that's about to happen, expect her to seek an area no one will see, and/or immediately gulp down one of those potions I mentioned. Oh, and speaking of Jekylls, they should be mainly the mad scientist type, with a bit of shyness and timidity to contrast with Hyde's bravery, sadistic nature, and...well...the 'I am going to rape you and you will love it' mentality of this part of the MG. Of course Hyde would be immensely strong.