Emerald is a nano-machine colony created by Kim, Fei's incarnation from the Zeboim era (4000 years prior to game events). She is a kind young woman (who transforms into an adult if you complete her sidequest towards the end of the game) and she is very infantile and not very well spoken. 

Comparable to Leah from Chrono Cross, Emeralda is a highly lethal and skilled fighter, but she is a dog on a leash for the most part. mid female range here.