Constance is a daughter of the High King and Harmony; sister to Miracle, Honor, Elegance and Destiny. Mira often refers to her as Costa. Constance's powers are strongest in Zeropolis, and therefore it is where Mira believes she is being kept. It is assumed that she's around thirteen or fourteen since Mira is eleven and Honor is sixteen.
When seen in the trial against Trillian, Constance is described as having long straight hair and a bored expression on her face. Another time is in book three, the Crystal Keepers where the Hunter and Cole finds her and a friendly seeming supercomputer.
In The Rogue Knight, Mira describes her sisters as being in relation to their names. Constance was the most level headed and reliable. In their 1st meeting with Roxie, Constance tried to attack them.
Constance, like her sisters had her shaping powers stole by her father at a young age and she remained that age after her powers were stolen.