Carla Medea

Carla Medea was a sister of Celena Medea. She presumably died by the time Brianna Barter encountered the ghost of Celena, since Celena referred to Carla in the past tense.

Since Brianna took the appearance and identity of Celena, with Celena's ghostly help, they decided to refer to the ghost as "Carla" for those few occasions when somebody perceived the ghost that was communicating with Brianna.