Hirahira to Gunjou

Kasumi is a cheerful girl and is loved by all her classmates, but she hides a secret: a butterfly-shaped birthmark on her ankle. Naoshi, a gloomy and lonely boy, and Kasumi's childhood friend, is the only one who knows this secret, but the two have not spoken for several years. Kasumi wants to be Naoshi's friend again, but he avoids her... Why?