From Sugar Oasis:

Riria is a middle school girl who is absolutely, hopelessly in love with the guy who lives next door. The only problem is that he is a LOT older than her. His name is Rinpei, and he is 26 and a businessman from a rich family. Although he is nice to Riria, he (rightfully) considers her just a kid, and just treats her like a friend. Poor Riria, she loves Rinpei so much, but he is so much older and doesn't see her as a real woman.
To solve the age problem, Riria transforms herself into a totally different person, trying to make herself look older. Now is the test, she decided to find Rinpei on the street and see if she can fool him with her 'disguise' into considering her a real 'woman' and see how he likes her then. Amazingly enough, things go well. Riria, in her new persona manages to make quite an impression on Rinpei! In fact, he really likes her!