Who the heck is Emeralda?
However, before she could be "born," the military invaded Kim's lab looking to seize Emeralda and use her for their own ends. Rather than give her up, both Kim and Elly died to save her. Emeralda remained locked away in stasis for 4000 years, until the ruins of Zeboim were excavated. At this time, the party tried to keep her out of Solaris' grasp, but they failed and she was taken to Krelian. He studied her and used the technology in her body to advance his research, then discarded her. She was sent to the third Solaris Gate to kill the party. However, she was defeated and was taken aboard the Yggdrasil, at which time she excitedly greeted Fei as "Kim." Emeralda saw Fei as her long-dead father (which was partially true, as Fei was a reincarnation of him), and exclaimed that she had been having a long dream but now she was awake and could be with Kim. After some explaining she realized that Fei was not actually Kim, but persisted in calling him "Fei's Kim" and joined the party. She is rather adaptive to her new environment, although it seems she doesn't catch onto the language very fast. Personally, I find this contradictory to her nature, especially since when she first joins the party her grammar is just fine. I prefer to think she's smarter than that. ;)
Emeralda is a beast in battle. She's fast, powerful and also wields elemental Ether attacks. Her fighting style consists of literally turning herself into a weapon. Her arms, legs, and even her hair form the shape of various weapons during her attacks. Emeralda also has the ability to turn herself into pure energy and summon a demon-like creature from another dimension. Her only downside is low HP. She pilots the Gear Crescens, which is a Vierge-class Gear with enormous wings/hands on its head in place of arms. It mimics Emeralda's performance in battle, being one of the most powerful and versatile in the game, but again suffering from low HP.
Links to Xenosaga
Please be aware that this section contains spoilers for Xenogears, as well as minor spoilers for Xenosaga.
Emeralda appears to be a young girl or a teenager, but she is actually a nanomachine colony designed 4000 years before the game takes place. In the country of Zeboim, humanity was dying out due to war and a genetic defect that rendered people unable to have children. To counter this, a man named Kim Kasim began work on a light of hope for humanity - a child made of nanomachines. He was motivated not only by social conscience, but by the fact that his wife Elly was unable to bear a child. His creation would be their daughter - their angel. This child was named Emeralda, for her mane of brilliant green hair.
At the end of the game, there is an optional area you can explore where an important event relating to Emeralda takes place. Exploring an old lighthouse, the party comes upon a ruined Zeboim city. During the exploration, Emeralda regains memories of her parents Kim and Elly, and what happened before she was born. She decides that she doesn't want to be a burden anymore and that she needs to grow up - and she literally does! She goes from looking like a child to a young adult in an instant. The transformation is purely aesthetic game-wise, as her stats remain the same, but it's an important turning point for her as a character. She chooses to stop being a naive child and face the world as an adult. Although this side quest is optional, the ending sequence of the game shows Emeralda in her adult form, so this event is considered canon to the story of Xenogears. I assume it was meant to be part of the story, but was unable to be fully realized and only made it in as a side quest.
If you've played the Xenosaga series, you might have noticed a character reminiscent of Emeralda. MOMO, the 100-series Realian, resembles her in a few ways. She is also an artificial human created for the benefit of mankind, and sees her creator, Joachim Mizrahi, as her father. Both Emeralda and MOMO are seen as the hope for the future, innocent and pure. They are both also pursued by an antagonistic organization in order to use the technology contained in their bodies. The Xenosaga Episode III database even says that MOMO's consciousness is able to be transferred to an adult body, so she can "grow up" and have children. MOMO is not as powerful as Emeralda however, and not as cool in my opinion. :P