Cocona Bartel is Croix's adopted little sister and a main character in both Ar tonelico 2: Melody of Metafalica and Ar tonelico Qoga: Knell of Ar Ciel. 2 years prior to the first game, Cocona lost her entire family in a IPD outbreak and was found and adopted by Croix.
In Ar tonelico II, Cocona and Croix are called on by Captain Leglius to investigate the IPD outbreak at the Rakshek Resort, where Lady Cloche was spending her vacation. Unfortunately, Cocona is revealed to be a Level 7 IPD infected Reyvateil and is taken away to the IPD Labs and Croix is forced to rescue her. She slowly becomes weaker and starts gradually losing her memories due to Infel Phira being hacked and Croix is forced to enter her cosmosphere to shut down her Boundary Gate and saving her life.
In Ar tonelico Qoga, Jacqli asks Cocona to take the small piece of the Heart of Gaea to Harvestasha in the Tower of Tyria in Sol Cluster. But since her Song Magic comes from Infel Pira, the Clustanians perceive her as a threat and attack. Injured, Reverand Gengai takes her to Dr. Hikari Gojo to be treated. With no where to go and the Clustanian Army still looking for her, she disguises herself as a boy named Tatsumi and hides out in Blue Canyon Hamlet, where she joins Aoto in his journey.
