Merida Island Base

In the novels, Gauron's twins were boys, their names Fei Jau and Fei Hung. Their most recognizable forms are from Full Metal Panic! The Second Raid as girls Xia Yui Fan and Xia Yui Lan. A third version of the twins can be seen in Full Metal Panic! Sigma. No longer Chinese, they are presumably Russian, and named Lena and Julia. In all of their versions, they retain the same basic history. They were found and raised by Gauron, who trained the two of them to be emotionless assassins, the very image that he had once glimpsed in Kashim when he was a mercenary trainer in the Middle East.

The twins are seen during the events surrounding Hong Kong, having recovered what was left of Gauron’s body as he barely survived, nursing him into some semblance of health, though there wasn’t much that could be done after he had used the self-distraction on the Codarl-i during his attempted hijacking of the Tuatha De Danaan.

Under Gauron’s orders, they followed the control of Amalgam until they were able to steal a Codarl-m for themselves, hiding in Hong Kong as Amalgam and Mithril searched for them due to the erratic pattern of destruction left in their wake. Fei Hung/Yui Lan/Julia was sent to Tokyo to kill Kaname.

Fei Hung was stopped by Kaname’s ingenuity, and later killed by one of Leonard’s Alastors for refusing to obey his orders to withdraw. Fei Jau/Yui Fan/Lena stayed in Hong Kong to continue destroying random destinations in order to drive the divided city toward civil war, and act that would have had a domino effect and possibly triggered another world war and angered Amalgam. In all versions, Fei Jau was stopped and the crisis was averted.

In the anime, Yui Fan was killed by Gates when he and Amalgam’s execution squad arrived, after he taunted her with Yui Lan’s dead body. Her breaking point came when Gates treated the corpse too roughly and broke one of her arms, claiming that ‘the god damned thing broke.'