Shidou, Kei

祠堂圭

Kei is Miki's best friend and classmate. The two were hanging out together at the mall when the outbreak happened, and sought shelter in a storage room. However, after awhile Kei becomes depressed and impatient with sitting around waiting for help; she leaves their improvised shelter on her own and gives Miki, who decided to stay behind, her portable CD player. After she is rescued by the School Living Club, Miki often wonders what became of her friend.
In the manga:

Spoiler