The only link to Sawako's past, Shino was her elementary school classmate and the only person who Sawako talks to as the series begins.
During elementary school, by mistake, Shino called Sawako "Sadako." When they attended the same middle school, the boys in their class, afraid by Sawako's appearance, heard rumors that she was known as "Sadako," and since then it became her nickname. It is because of this, that Shino feels guilty about the rumors circulating over Sawako that started from her calling her "Sadako." Thus she cares about Sawako and whether she is fitting into high school. Shino is in a different class and regrets that she doesn't get to see Sawako very much. She is a cipher, appearing only occasionally as a catalyst for Sawako's interactions with other people. Perhaps for story reasons, Sawako is too modest to consider her as a friend.
According to Shiina, Shino was an unnamed character in the manga, but was named by the staff for the anime. Shiina also wrote that "Shino-chan in the anime is really cute."