In regards to Natsuki, I had never heard the term "tsundere" before. Now that I've looked it up, she certainly doesn't leave the cold hostility for very long. Which made her less apealing as a character to approach for me.
With Yuri, I didn't notice the over sensativity, if anything, I enjoyed the mystery about her. I thought Natsuki had a hair-trigger sensativity with almost everything though. With respects to Yuri in act 2, I can't really blame the Yuri character for what happens considering all of the changes that are forces onto her and the others. I actually tried my hardest to see if act 2 would have similar results to Natsuki as well. Regardless if you focus your poems on her in that act, it still goes back to Yuri, which did lead to some interesting new converstations but no interactions. I thought a focused, act 2 Natsuki would be more [forceful] or so.