Shizuka Kikuchi

Another of the more prominent "background characters," Shizuka Kikuchi was actually introduced as a one-joke character. When Nube does a class on the Ichimatsu Doll, he ends up using a cute girl with long black hair (Shizuka) as part of a joke to get rid of a possession. Outside this early joke, this female typically is found in the background, either doing things alongside the rest of the class or hanging out usually with her best friend Noriko. But while she seems merely innocent in the background, a later chapter reveals that it allows her to easily tattle on her classmates to Nube, due to a strong sense of justice that she holds that she puts above her friends. (It nearly gets her in trouble with a dangerous team of youkai.)

(Source: Wikipedia)