Shino is a close friend of Saki Saki. They are in the same class and the same after school club. Shino hasn't appeared much for most of the adaptation but she's a very serious and considerate girl.
SpoilerAfter learning Naoya and Saki Saki were both on a two-time relationship with Nagisa, she decided to expose their relationship, and attempt to end it, in order to help Saki, believing it was hurting her feelings. However she also has feelings for Naoya, and started to have doubts about hiding her feelings or not.