[houkoholic]

Originally Posted by Hyper

At this point I certainly don't convinced that Nibutani is going for Yuuta...yet. Her reaction to the indirect kiss scene at the beginning doesn't go well with that theory. I hope we'll find out soon why she joined the club.

If you piece together the clues it seems fairly obvious:

Spoiler for clues and speculations:

Dekomori in this episode said she got this (massively chuuni) book two years ago from the "magician" named Mori Summer (as pointed out Mori/forest=森　Summer/natsu= 夏 hence combine the two kanji you get 森夏 = Shinka).

And from the chuuni character description above taken from the official site Shinka is "fighting Rikka's servant (ie Dekomori)" and "looking for the past legacy" so if Mori Summer = Shinka then this implies she's going to try to get her book back from Dekomori.

From episode 2 Shinka also said she's just pretending she is dependable and all that etc so the obvious conclusion is that Shinka is like Yuuta who wants to bury her chuuni past and hide the fact that she used be a chuunibou and begin to live a new normal life when entering high school, and because that book is the legacy of her chuunibou she can't let someone know how bad she was and wants to take it back, thus the way to do it is to join the club so she has more chances to get close to Dekomori.