Cosette's vocal coach, Eldritch Valdemar, told her she was a failure but asked her to look into his eyes. When she did, he placed in her an alien virus which made her a fabulous singer — and screamer. The Eleventh Doctor and Amy went to see her at the opera. As they left, her ex-boyfriend Louis chased after her, asking her what was wrong. When the Doctor, Amy and Louis broke into Monsieur Valdemar's house, Valdemar had Cosette sing/scream them into unconsciousness.
Later, Louis held Cosette to prevent her from following Valdemar. The Doctor had her carry him while chasing her master. They went to the Notre Dame de Paris, where the Doctor rang the bell, releasing Cosette and the other victims from Valdemar's control. They then drove him over the edge of the roof, killing him. (COMIC: The Screams of Death)
Cosette was absorbed by the TARDIS when she touched it, and it departed without her return. An accelerated temporal field them resulted in the infected TARDIS mutating, and Cosette appearing on the surface. The victims of the TARDIS then coalesced into one huge creature, which was then lured into the teleport by the Doctor in an attempt to restore the individuals. This resulted in the creation of Chiyoko. (COMIC: Apotheosis)
Later, in the Doctor's final confrontation with Chiyoko, the Doctor brought her, along with Margaret and Konami, to the Museum of Lost Opportunities, endangering Chiyoko's existence and thus limiting her powers. Upon seeing the lives that she was taking away, Chiyoko undid her own existence, sending Cosette back to Louis in 1858. (COMIC: The Child of Time)