Gomen ne Hatsukoi

Year 3 of high school, winter. Chiho and the friendly Sousuke kindly accept Mahiro, an out-of-season transfer student. Chiho and Sousuke are childhood friends who live next door to each other. They are not family, but they are special to each other—and Mahiro yearns for their relationship. These feelings eventually turn into her first love for Sousuke...

(Source: Kodansha, translated)