For me, Shinka is the most fascinating character in the cast. Precisely because of how much her character develops and transitions.
I find myself asking "What's the mask, and what isn't?" about Shinka in much the same way I ask such questions about Clark Kent/Superman and Bruce Wayne/Batman.
So here we have Shinka Nibutani/"Mori Summer".
I think that "Mori Summer" is a reflection of the romance idealist lying at Shinka's core. As "Mori Summer", Shinka gave full vent to that. However, that ended up causing her all sorts of extreme embarrassment. She hence became jaded about her previous romance idealism, and embraced a "more adult" approach to romance and sex. At the same time, though, she wanted to develop a persona that would ensure others admire her and respect her, to fully get over the embarrassment of the past.
So she became the perfectly sweet and perfectly responsible class rep that we saw in Episodes 1 through 3. Maintaining that perfect image is stressful, though, so she finds a good outlet for the stress in the other members of Rikka's club. They already know about her embarrassing past, so she feels no need to maintain the perfect image around
them.
By Episode 9, the positive psychological benefits of having that outlet has resulted in a more balanced Shinka - Not the
perfectly sweet girl of Episodes 1 through 3, but nor the very abrasive girl of Episodes 4 through 8.
Shinka has moved beyond the extremes to a more well-rounded whole. And in so doing, she has, like Yuuta, allowed herself to indulge a bit in her past chuunibyou persona (by playing matchmaker for Yuuta and Rikka, and using a shoujo manga to do it).
And yes, I do think she has a strong sense of personal responsibility, which comes through each of her conceptions of self.