Chelsea

Name             : Chelsea
Alias               : Charmion (Original Name)
Species           : Vampire
Perpetual Age : Unknown
Clan                : Volturi Clan - Guard
Created By     :  Aro
Height             : 5"3

Hair Color       : Light Brown
Eye Color        : Red
Skin Color       : Pale

Chelsea is a vampire and a permanent member of the Volturi guard. She is first mentioned in Breaking Dawn. She has the supernatural gift of being able to weaken and strengthen the emotional ties of relationships. She is mated with Afton, also in the Volturi guard.

Early Life

Chelsea, originally named Charmion, was born in Greece, before 1100 BC, though not long after the original Volturi members. She changed her name a few times over the millenia to keep it from attracting attention while frequently interacting with humans and vampires outside the city of Volterra. In the 1950's, she changed her name to Chelsea, and will use it until it becomes out of fashion.
Aro discovered her in the midst of a difficult time for the Volturi, since his brother-in-law, Marcus, failed to find the killer of his wife, Didyme, and became suicidal. Aro had Chelsea use her power to tightly bind Marcus to the Volturi. Since then, Chelsea became the linchpin in the Volturi as she keeps the other guards unified and the coven more functional. Because of her vital role, she often gets what she desires from Aro, who also uses Corin's gift to keep her in the Volturi.
Much of her history remains unknown throughout the series. However what is known is that Aro used her gift to bind Marcus to himself and Caius (thus making him remain loyal to the Volturi). With this it can be assumed then that Chelsea was with them when the Volturi first began and later rose to power.
Her role in the Volturi is explained in detail by Eleazar in Breaking Dawn: in battle she separates the allegiances between allied covens, thus preventing them from fighting as a whole, allowing the Volturi to defeat them much easier. She also distances the innocent members in a coven emotionally from the guilty, thus justice without unnecessary brutality.