Rika is 10 years old and first appears in episode 1. Out of all Sakura's friends she is the most mature. She also has a big crush on her teacher Mr.Terada. In the Manga her and Mr.Terada are engaged to be married once she comes of age.
A little insite into the world that is Japan. Unlike American laws on marrage, Japan has different laws for it. While America has it so you can only marry when your 18 or older, in Japan you can get married at the age of 16.