Kyou no Kerberos

Kyou no Kerberos summary:

From MangaHelpers: Ever since an childhood incident, Chiaki has never been able to truly feel joy or have fun. One day, his traveling father sends him a mysterious cardboard box. Once opened, a girl who calls herself Cerberus pops out of the box! This girl, who has three personalities in one, vows to protect Chiaki from all harm. Will Chiaki be able to live with this three-in-one girl? [DrCoke]