Shugo Chara! Vol 6

From the Kodansha edition

BETRAYAL Amu has three Guardian Characters who represent different sides of her ideal self–and a fourth who has gone over to work for the evil Easter Corporation! Amu must now face her renegade character while the Guardians try to foil Easter’s sinister new scheme–all while dealing with a surprise traitor in their midst! This volume of Shugo Chara! includes special extras after the story!

Source: Anime News Network Encyclopedia