Housoushitsu no Kigurumi Girl

Nao Kurusu transferred to an all-girls school in the countryside after her father's job move. While she was trying her best to get used to the new school, she runs into someone in her class wearing a costume. Why are there costumes at school? Who is inside this costume? Nao is curious and tries to find out their identity but...!? (Source: Official site, translated)