Davida and Shirly are two young members of the cirque who assist multiple acts like, Mr. Crepsly and the Wolfman. They are professional hypnotist who traveled with the cirque for many years. In the Cirque du Freak manga they are shown to be able to play the flute, as they do while Mr. Crepsly and Madam Octa are preforming. After the first book these two were not mentioned again until The Sons of Destiny where Darren mentions that they left the cirque shortly after he arrived saying that the life of a freak was not the life for them. In the Cirque du Freak manga volume 2 Darren talks about all the friends he made at the cirque, when they're shown either Davida or Shirley is amoung the group meaning they hadn't left yet by the Vampire's Assistant.
The Cirque du Freak
The Sons of Destiny
The Cirque Du Freak manga volume 1
The Cirque Du Freak manga volume 2