Kouta's closest friend. She has asthma.

Spoiler

Later on, Kouta's friends grow further and further apart due to Mai's growing relationship with him, until Mai convinces him to let him play soccer with his friends. During this time, a heavy rain begins pouring down and Mai is alone caring for Blue. Leaving her handkerchief behind as a blanket for blue, she struggles to get home and collapses on the way, presumably dieing as Kouta witnesses her being driven away in an ambulance.

Spoiler

Momo states to Kouta that Mai's soul is still on earth for she still had regrets. Mai's soul meets with Kouta as she makes him promise to care for their "child", Blue. She then passes away, finally free of any regrets and ties to earth.