She is a classmate of Hiroaki.  She is also Sadako, the person who has been writing letters to Ryutaro. 

Episode 1 
Kakushi Dere 
Nonoka goes out of her way to make a written confession to the older brother of one of her classmates, but he doesn't even read the letters. This prompts her to stop by in person, and express her "hate" for him through various antics... 
 
Episode 2 
February 14 
The second episode is about Makoto and his step-sister Misaki (who is about as tsundere as you can get). Misaki secretly has feelings for Makoto and one day is accidentally caught by him while masturbating in his room. Despite Misaki being an extreme tsundere, they finally both convey their feelings to each and have sex. 
 
Episode 3 
Ubu x Ubu Syndrome 
Taiki confesses to his highschool crush, Mihashi Chizuri, not knowing about a nasty rumor between her and Ikeda-sensei. After hearing about the rumor and confronting his now-girlfriend about it, Taiki is relieved to know there was no truth to it. 
The couple then have sex for the first time, although throughout the episode Taiki had been fantasizing about his girlfriend in various sexual situations.

1. Kakushi Dere (Hidden-Dovey) 
2. Dare Koi Train (Who's Riding the Love Train?) 
3. Isshoni! (Together!) 
4. February 14 
5. Ubu x Ubu Syndrome (Beginners Syndrome) 
6. Hana to Buta to Teishoku (Dinner Between the Flower and the Pig) 
7. Sys☆con 
8. Miyuki: Manga-dou 
9. Uwaki na Tokoro 
10. Dere Kakushi (Hidden-Dovey Extra)

Nonoka Nonoura