Midori and Hikari are partners. They are on the same group in the final match of the eleventh East-West War. When Hikari is caught off guard, Midori is willing to protect her. Apparently the two can perform a combination technique, called the "Hipp Train".
Nanase addresses her by her first name, Midori. The two seem to be roommates since Nanase knows that Midori keeps snoring while sleeping.