The "other girl" is only shown in the OP and in the preview approaching Nanaka with a book. She wasn't part of the original group (as Yuki said), but she appears to be friends with Nanaka.

It shows promise. There's hopefully some deeply rooted issues in this series. I've had the problem of not recognizing girls/friends/etc I haven't seen in years (who can apparently recognize me in an instant), but the full slap means Sana has done something much worse. I imagine it's got something to do with what the twin was trying to yell at him on the train. However, Nanaka avoided him at the shrine too. Perhaps she just wasn't sure it was him yet.

Aoi's voice is painful. I like Tomoko Kaneda, but the sound of her voice is really, really irritating when it's not in the body of a 12 year old or younger. (Chiyo from Azumanga, Gretel from Black Lagoon, the loli from sola, etc). It's just wrong, especially in when her character is that well endowed. If she was the main female lead and getting the most screen time, I'd probably drop this show right here.

And if the last scene in the OP turns out to even come close to actually occurring in the show, I'll drop it then too.

For now, I'm intrigued by Nanaka's rather violent reunion, contrasting her shy scene at the temple.